Disclaimer: Information may change as research develops.
When I say that a horse has the possibility to produce a certain offspring, that depends entirely on what the genetics of the horse you are breeding it to are.
For example, if you breed two black horses (Ee/aa and/or Ee/aa), you will not miraculously get a Bay (Ee/Aa). But if you breed a black horse (Ee/aa) with a chestnut horse (ee/Aa) you absolutely could get a Bay (Ee/Aa), and so on.
Foal color is determined by both parents genetics. With dominant genes (EE, AA, CrCr, GG, etc.) one copy of that gene will always pass to it's offspring.

"Grandfathered In" - What does it mean?
Sometimes genetic information comes out that contradicts previous assumptions. Therefore, our group standards for colors and markings may change over time. That means that during the present, there may be horses who are registered in our group that if they were to be re-submitted as a new horse to the group, they would be declined based on the inaccuracies of the design. However, the moment a horse is approved into this group, their registration is approved so long as they have a reference registered to us. As long as at least one reference (foal or adult) is registered to our group, that horse can make as many new references as they want and still participate in our group as a fully registered member. That is because their design was "Grandfathered in." Their design was approved into the group at a time when it was acceptable. And as the times changed, and new genetic information came out, these designs are NOT kicked out of the group, they are allowed to stay. HOWEVER, all of their subsequent foals must meet our groups registration standard at the time in order to be approved for registration. It is only when a horse has no registered reference with our group that their design will be re-evaluated for registration. 

Eyes

Base Colors

Black "Ee or EE"
Black based horse = any horse with Ee or EE in its genetics, including Bays
Ee/aa This horse has the possibility to produce Black or non-Black offspring
EE/aa This horse will ONLY produce Black or Bay offspring


Chestnut "ee"
Red based horse = any horse with ee in its genetics
ee/aa This is the most popular genetic possibility for a chestnut. This horse has the possibility to produce Red based offspring.
ee/Aa When bred to a Black based horse, this horse has the possibility to produce bay or non-bay offspring
ee/AA When bred to a Black based horse, this horse will ONLY produce Bay offspring (if the foal turns out Ee)
ee/Ata When bred to a Black based horse, this horse has the possibility to produce Seal Bay offspring
ee/AtAt When bred to a Black based horse, this horse will ONLY produce Seal Bay offspring (if the foal turns out Ee)
ee/A+a When bred to a Black based horse, this horse has the possibility to produce Wild Bay offspring
ee/A+A+ When bred to a Black based horse, this horse will ONLY produce Wild Bay offspring (if the foal turns out Ee)


Liver Chestnut / Sooty Chestnut "ee with or without any combination of A, and with nSty or StySty"
Liver Chestnut is the darkest shade of chestnut, and can reach a near-black color. The mane and tail can be a regular red/chestnut color (unless flaxen is present, which will cause blonde hair unless covered by the sooty), or the mane and tail can be partially or fully darkened from sooty.
You can see IRL examples of Liver Chestnuts here: 1 - sooty expressed through the darkened dappled areas), 2 - this one is supposed to be flaxen, but its mane and tail are darkened by sooty), 3 - combined mane/tail/body expression, including neck shadow and darkened hip + legs countershading, 4 - combined mane/tail/body expression, including neck shadow and darkened hip + legs countershading, 5 - sooty tail and darkened body.

Bay Colors

Bay "Aa or AA"
Ee/Aa This horse has the possibility to produce Bay or non-Bay offspring
Ee/AA or EE/AA When bred to a black based horse, this horse will ONLY produce bay offspring
There are many shades of bay (Dark Bay aka Mahogany Bay is the darkest. Blood Bay is the second darkest, and appears reddish. Copper Bay is typically more bright, and appears orange red. Standard Bay is the typical Bay. Light Bay aka Golden Bay is the lightest, and can appear anywhere from light red to a golden yellow), but these are just words to describe the shade of bay. All of these examples are defined as "Bay" and are genetically the same color, even if one has a darker or lighter shade than the other.

Seal Bay "Ata or AtAt"
Seal Bay is the darkest of the bay colors, as it is the least restrictive to black points making most of the horse dark with sparce light "seal" points.
The color Seal Bay was nicknamed as "Brown."
Ee/Ata This horse has the possibility to produce Seal Bay or non-Seal Bay offspring
If bred to an AA Bay, the foal will be Bay. If bred to an Aa Bay, the foal will be Seal Bay
EE/Ata this horse will ONLY produce Bay or Seal Bay offspring
If bred to a horse that is AA for regular Bay, then the foal will be regular Bay, otherwise, the foal will be Seal Bay
Ee/AtAt This horse will always pass the Seal Bay gene (depending on the other parent, the foal will either be AtA or Ata)
EE/AtAt This horse will ONLY produce Seal Bay offspring.
The body and legs of the Seal Bay horse should be the same color, with lighter brown areas around the belly in the least, with the lighter areas also sometimes reaching the eyes, throat, and buttocks.

Wild Bay "A+a or A+A+"
Ee/A+a This horse has the possibility to produce Wild Bay or non-Wild Bay offspring
If bred to any Bay or Seal Bay, the foal will be Wild Bay
If bred to any Black horse, the foal will either be Black or Wild Bay
EE/A+A This horse will ONLY produce Wild Bay offspring
The shade of the base is light, with the black on the legs restricting to the joints on the horses legs, instead of fully black legs.

Cream Colors

Palomino
ee/aa/nCr
Palominos are golden in color with blonde-white mane and tails.

Chocolate Palomino / Sooty Palomino
Any Palomino Genetics + nSty or StySty
Chocolate Palomino is caused when the Sooty gene attaches to ee/nCr. This causes the golden shade of the body to darken either overall or in a dapple pattern, whereas the mane and tail may or may not remain fully or partially blonde (Sooty does have the ability to partially or fully darken the mane/tail, but doesn't always).
You can see IRL examples in order of expression from minimal to maximal here: 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10  

Cremello
Any Chestnut Genetics + CrCr
For Example, ee/aa/CrCr , ee/Aa/CrCr
Double Cream (CrCr) causes blue eyes.

Buckskin
Any Regular Bay Genetics + nCr
Ee/Aa/nCr , Ee/AA/nCr , EE/Aa/nCr , EE/AA/nCr , and the same for AAt (Regular Bay, but also a Carrier of Seal Bay).
The cream gene dilutes the brown hairs of the Bay base to a golden color. The horse will retain the black points around it's eyes, muzzle, legs, and it's mane and tail will remain black.
There are varying shades of the Buckskin horse color- including Pale, Standard, and Golden.
Pale Buckskin is also called "Buttermilk Buckskin," or "White Buckskin," and is the lightest shade of tan, see examples here: 1 , 2 , 3
Standard Buckskin is a browned tan, see examples here: 1 , 2 , 3 , 4
Golden Buckskin is a golden tan, see examples here: 1 , 2 , 3 , 4 , 5

Seal Buckskin
Any Seal Bay Genetics + nCr
Ee/Ata/nCr , Ee/AtAt/nCr , EE/Ata/nCr , EE/AtAt/nCr
Seal Buckskins don't present the same phenotypically as regular buckskins, as their Seal Bay base has less brown hairs available for the Cream gene to dilute.
You can see an IRL example of a Seal Bay (left) vs Seal Buckskin (right) HERE. 

Wild Buckskin
Any Wild Bay Genetics + nCr
Ee/A+a/nCr , Ee/A+A+/nCr , EE/A+a/nCr , EE/A+A+/nCr , and the same for A+A and A+At (Wild Bay, but also a Carrier of Regular or Seal Bay).
 Wild Buckskins look like regular Buckskins (Pale, Standard, or Golden), but their black points are restricted on their legs, just as with Wild Bay.
You can see some IRL examples here: 1 , 2 , 3 , 4

Perlino
Any Bay Genetics + CrCr
For Example, Ee/Aa/CrCr , Ee/A+a/CrCr
Double Cream (CrCr) causes blue eyes.

Smoky Black
Any Black Genetics + nCr
For Example, Ee/aa/nCr , EE/aa/nCr

Smoky Cream
Any Black Genetics + CrCr
For Example, Ee/aa/CrCr , EE/aa/CrCr
Double Cream (CrCr) causes blue eyes.

Modifiers

These colors are added to the base color of the horse (Any color above)

Gray
Gray horses are born their base coat (usually with a gray ring around their eyes) and as they age they begin to gray out. Generally, the older a horse is the more gray it will become until it is "White Gray." White Gray horses are NOT true white horses, and can be distinguished by their dark skin (they will have dark muzzles whereas true white horses will have pink muzzles). The dark muzzle of a Gray horse will turn pink if a white marking (blaze, snip, etc) crosses through it. White markings always go on top of Gray, Gray does NOT cover up white markings.
nG This horse has the possibility to produce Gray or non-Gray offspring
GG This horse will ALWAYS produce Gray offspring

Grays can come in different patterns: Dark Dapple Gray (young-adult horse), Dapple Gray (young-adult horse), Light Dapple Gray (adult horse), White Gray (adult-senior horse), Fleabitten Gray (closeup), Gray w/Bloody Mark (example #2 , #3), Mulberry Gray

Gray horses can commonly become Somatic (part of the gene turns off) in three different ways:
Fleabitten Grays will have tiny flea bite-sized flecks missing from the Gray where the base coat shows through. None of these affected areas for these horses will ever fully Gray out.

Bloody Mark(s) (also known as Bloody Shoulder Markings, though TS calls them "Bloody Mark(s)," as they can appear on all areas of the body, not just the shoulder) are large patches of the base coat that show through under the Gray. None of these affected areas for these horses will ever fully Gray out.
    This prized marking actually has quite the history! Legend says that there was a brave heavily pregnant gray Arabian mare (Arabian mares were often chosen as mounts in war over Stallions) whose rider (supposedly a chieftain) was mortally wounded by pursuing robbers. The mare outran the robbers and loyally returned to her riders tent. When his body was removed from her back, his dried blood had created a red streak running down her shoulder. It's alleged that very night she gave birth in the tent to a gray colt - the first to be born and mature with a rusty red mark running down his shoulder. So it is according to legend, that every horse with this marking descends from that great Arabian mare. As Arabians helped found the Thoroughbred breed, this marking sometimes pops up in modern Tbs.

Mulberry Grays are horses whose manes and tails will never fully gray out, often sporting their original base color.

Gray Inhibitor Gene
It is theorized that there is a gene responsible for slowing down the progression of gray in select breeds (primarily Iberian breeds, though Thoroughbreds are included). It is heritable, though it doesn't display on homozygous grays, nor non-grays, it only displays on heterozygous grays. The horse does gray as it ages, just very slowly, and will still have much of its' base coat present well into its 20's, and it will never fully gray out to White Gray. Here is an example of a PRE stallion w/Gray Inhibitor at 10 years old (headshot). Comico IV was a PRE Stallion w/Gray Inhibitor (also seen here). Here is his brother, the similarly named Comico VI, who also appears to have the gene.
Gray Inhibitor can only affect nG horses. It can be carried by horses who are GG or non-gray, but will only show on horses who are nG. 
For the sake of HARPG, I have denoted this gene as nIh or IhIh.

Chubari Spots
Similar to Birdcatcher Spots, but much larger, these white egg shaped (and generally egg sized) spots are also nicknamed Terarch Spots. These spots only appear exclusively on horses with the Gray gene. There is no genetic code to define these spots as they are not hereditary, they simply appear or do not as a sort of pattern of Gray.

Flaxen
ff This horse will only produce other flaxens when bred to another (ff) horse.
Ff This horse is a flaxen carrier, but does not physically show it. If bred to a Flaxen (ff) horse, there is a 50/50 chance the foal will be Flaxen (ff) or a Flaxen carrier (Ff)
Flaxen will only show on Red based horses, but may be carried by Red or Black based horses.

Sooty
nSty This horse has the possibility to produce Sooty or non-Sooty offspring
StySty This horse will ALWAYS produce Sooty offspring
Sooty has a wide range of expression, and is expressed in generally three potential ways: 1) optional mane and/or tail darkening, with or without either 2) dark dappling and/or 3) countershading (smooth/non-dappled darkened areas of fur). Both dark dappling and countershading like to focus around the neck (called a "neck shadow"), areas of the face, topline, shoulder, hip, and legs.

Normal Facial & Leg White Markings

Genetic White Markings

Birdcatcher Spots
These white spots are generally the size of a quarter at largest, and circular in shape. They tend to pop up in family lines (even sometimes skipping generations at a time), which leads us to believe that they are genetic to an extent. For this reason, if your horse has an ancestor with Birdcatcher Spots, you are welcome to have them have the markings even if their direct parent did not (to simulate the gene popping back up as it does in real life). Just be sure to add nBcr to their geno. Realistically, it's more likely that the gene works on the basis of recessive heritance (like flaxen), but I do not know for sure, so for the time being, TS will require nBcr or BcrBcr in the geno. However, a great/grand foal can still get Bcr from the ancestor even if they didn't pass it onto their foal/your horses' parent.
nBcr This horse has the possibility to produce offspring with Birdcatcher Spots, or not
BcrBcr This horse will ALWAYS produce offspring with Birdcatcher Spots

Frame Overo (aka "Frame" or "Overo")
nO This horse has the possibility to produce Overo or non-Overo offspring
OO can happen if two Frame Overo horses are bred together (25% chance). This is called Lethal White, and will result in the death of the foal.
When you breed two Overo horses together, it does not always result in a Lethal White foal. In fact, when breeding two nO horses together; there is a 50% chance of producing a healthy nO Overo offspring, there is a 25% chance of producing a healthy non-Overo offspring (foal will not carry Overo at all), and there is a 25% chance of an OO Lethal White offspring (foal will die). Therefore, in breeding two nO's together, there is only 1/4th chance the foal will be Lethal White. Odds are (1/2 chance) the foal will be a healthy Overo, or 1/4th chance the foal will be a healthy non-Overo.
This gene can cause blue only in one eye (one blue eye, one brown eye), or parti-blue eyes (half blue and half brown in one or both eyes).
When mixed with Sabino, the horse can have two blue eyes
White markings will never cross the topline. However, when maximally expressed, as seen here, a small portion of the marking may cross the neck (ONLY the neck, never the back or rump), turning the hair white.

Manchado
There is a lot of controversy centered around this color. It was thought to be the result of regular white markings (Sabino, Frame Overo, etc) being mutated before birth due to atmospheric properties found in Argentina, where the color was originally discovered, and it was thought to be purely the result of such, without the ability to pass down the color to offspring. However, speculations rose when a Hackney stallion was born with the coloration, whose Grand Dam also carried the color. Here is the section on the color from Dr. Sponenberg’s Equine Color Genetics (Third Edition): “The repeatability of the manchado pattern suggests a genetic cause, though the range of breeds in which it occurs is awkward because they are not related nor are they commonly crossed one with the other to produce breeding stock. Paintings of Hackney horses from the 1800s suggest that the pattern has been around at least since then, if only rarely. The sporadic occurrence of manchado suggests that it might be due to a recessive mechanism, and moreover that the allele is rare.”
For the sake of HARPG, and based on my own beliefs, I am going to side with the idea that it is genetic, but can only be showed physically through it's dominant form. Therefore, here we go:
MchMch means the horse carries and physically displays the gene (compare to ff flaxen). This horse will only produce other Manchado horses when bred to another (MchMch or nMch) horse. If the MchMch horse is bred to a nMch, there will be a 50% chance of a MchMch foal (does show Manchado), and a 50% chance of a nMch horse (Manchado carrier, but does not show the gene).
nMch means the horse carries the gene but does not show it physically (compare to a Ff flaxen). If bred to a Manchado (MchMch) horse, there is a 50/50 chance the foal will be Manchado (MchMch) or a Manchado carrier (nMch). If bred to a non-carrier of the gene (mchmch), there will only be a 25% chance of producing a carrier (nMch), a 75% chance of producing a non-carrier (mchmch), and a 0% chance of producing a MchMch.
mchmch means the horse does not carry the gene (does not need to be added to your horses info, compare to FF flaxen).

Rabicano
nRb This horse has the possibility to produce Rabicano or non-Rabicano offspring
RbRb This horse will ALWAYS produce Rabicano offspring
Rabicano can be minimally expressed through no Rabicano body markings, but through a 'Skunk Tail' (see examples here: X , X , X). Remember that this does NOT cause the whole tail to be white! Skunk Tails can also be present with regular Rabicano markings.
Irl examples include SomethingAboutYou, Ho O Samantha, and Renoir.

Roan
nR This horse has the possibility to produce Roan or non-Roan offspring
RR This horse will ALWAYS produce Roan offspring
The Roan mutation for Tbs was founded by Catch A Bird. Irl Examples include Slip Catch, Lavender Fields, Lilac Hill, Pink Floyd, and Mathemagician.

Splash
nSpl This horse has the possibility to produce Splash or non-Splash offspring
SplSpl This horse will ALWAYS produce Splash offspring
This gene causes blue eyes almost every time, brown eyes are rare but can occur.
When SplSpl, Splash can cross the entirety of the back and rump. Dominant Splash can also cross over the neck in some areas, though not the full neck.
Splash can cross one or both ears, but the horse will be deaf in the marked ear(s).
Splash can cause a tail that appears to be dipped in white paint (similarly to Gulastra Plume), or can cause a fully white tail even if no white crosses the horses dock.
There are two different identifiable Splash genes found in purebred Tbs. The irl names for the genes are SW1 and SW8. While SW1 is commonly found in many horse breeds, SW8 is only found in Tbs. -UC Davis
Irl examples include Blue Eyed Angel and Reply.

Sabino *WE NO LONGER ALLOW THE REGISTRATION OF SABINOS AS PUREBRED*
* It has been determined that irl Tbs do NOT carry SB-1 (Sabino), but rather various W (White Spotting) genes. For TS Registration, we will give everyone the option to change their Sb registered horses to a W gene (even if they have in-game lineage) *
nSb This horse has the possibility to produce Sabino or non-Sabino offspring
SbSb This horse will ALWAYS produce Sabino offspring
Sabino can be minimally expressed through no Sabino body markings, but through a 'Gulastra Plume' - white hairs that mix in with the tail. Typically, the base of the tail is it's normal color, with the middle and tip of the tail being lighter. Gulastra Plume can also be present with regular Sabino markings. Remember that this does NOT cause the whole tail to be white!
This gene can cause parti-blue eyes (half blue and half brown in one or both eyes).
When mixed with Frame Overo, the horse can have two blue eyes

White Spotting (previously known as "Dominant White")
nW This horse has the possibility to produce White Spotted or non-White Spotted offspring (there are different W genes with different expression, denoted by different numbers - irl examples of these differences in expression are linked and outlined below).
WW can happen if two White Spotted horses are bred together (25% chance). This is called Lethal White, and will result in the death of the foal. Only W20 may be homozygous.
When you breed two White Spotted horses together, it doesn't always result in a Lethal White foal. When breeding two nW horses together, there is a 25% chance of non-White Spotted healthy offspring (foal will not carry White Spotting at all), 25% chance of WW Lethal White offspring (foal will die), and a 50% chance of nW healthy White Spotted offspring.
White Spotting is caused by a mutation in the KIT gene (KIT is the gene responsible for Sabino and Roan. Tobiano is very close to KIT, and while it isn't on KIT, it can disrupt KIT gene functions). Horses can generally only ever have TWO KIT gene mutations, though it's theorized that having more could be a possibility. Learn more here. However, because this is HARPG and because it is ~technically~ possible, we will not be enforcing the this.
If you aren't sure which type of W gene your horse is, please reference the irl examples of expression below. If you are still unsure, I will be happy to assist during the registration process. 
Here are the W genes that we accept in purebred TBs:
W2 - Descendants of KY Colonel (1946 Stallion). W2 can cause minimal leg white, sabino-like white spotting (as seen in World O'beauty), near-white coats (as seen in Beauty 'n Motion), or pure white coats (as seen in War Colors , White Beauty, Precious Beauty, and Patchen Beauty).
W5 - Descendants of Puchilingui (1984 Stallion). W5 can cause high irregular leg and face white (as seen in Paint The Dancer, Puchi Trap, Tahitian Moon, Artic Dazzle, Briarleyhillegacy, and Wild By Design), sabino-like white spotting (as seen in Sato, Puchis Pride, Black Oaks Paris, Embellished Lace, Jagged Illusion, Rambos Hotshot, Rambos Proof Positive, Colour Conspiracy, Picasso Twist, Red White 'N Blaze, and Simba Twist), and sabino-like roaning (as seen in Puchi Doll and R Secret Formula), near-white coats (as seen in Arctic Squaw, Arctic Color, Shew, Bright White, Puchis Rambo, Mystical White, Saton And Lace T B, and White Pharaoh). W5 can cause pure white coats when paired with W20 (as seen in Mysweetestdownfall, Padrama, Punz, B C F Headliner, and Cloud Ten).
    Side Note: There is a Puchilingui descendant with very unique markings named PrideAndPrejudice. However, I personally believe her markings came from her sire named Spot Pocket (I believe he carries an undiscovered W gene), or a combination of the genes. Spot Pocket has sabino-like white spotting, and out of his 14 foals thus far, 2 are white spotted: PrideAndPrejudice and Spot Me In Vegas, though many show high irregular leg and face white (as seen in A Spot I Got, Birds In My Pocket, and The Worlds A Stage). PrideAndPrejudice produced a pure white foal named Cielo Arctic Chill with the W22 stallion Arctic Cielo.
W6 - Descendants of Marumatsu Live (2004 Mare). The potential range of expressiveness is not yet known, since Marumatsu Live was retired in 2010 and she has yet to breed.
W7 - Descendants of Turf Club (2005 Mare). Turf Club has sabino-like white spotting with sabino-like roaning. She produced a filly in 2013 named PassionForFashion (featured with her dam in pic - you can see her base coat on her ears, therefore she is considered near-white, NOT pure white). PassionForFashion went on to produce a near-white colt in 2017 named Hopes Great White (featured with his dam in pic), unfortunately and quite tragically, it's been reported that he passed away in a fire at Clovis Crane's stables in March 2020, and was never bred. Therefore, the current known expression for W7 is sabino-like white spotting with sabino-like roaning, and near-white coats.
W12 - This gene was found in one single Unamed Thoroughbred colt who reportedly had about half body white. He was born and died in 2010, which means he never passed on the gene.
W14 - Descendants of Shirayukihime (1996 Mare). These horses will usually be pure white, however some spots of color may be present. 10/11 of her offspring so far have been white, with the exception of one Gray. Of the 10 white offspring, 7 were pure white and 3 were white spotted. The spotted offspring (for reference) include Marble Cake , Buchiko , and Butchini. Most of Shirayukihime's descendants are pure white. However, her spotted descendants (grand offspring and beyond) include Latte Macchiato, and Oeufs A La Neige's unnamed filly.
W20 - W20 on it's own can cause normal or slightly irregular leg and/or facial white, with or without a belly spot (featured in the image is a Warmblood named Fred who tested dominant for W20, he also has birdcatcher spots). When paired with a genetic white (Splash, Overo, or another W gene), W20 expands the white. Out of all of the W genes, W20 is the ONLY one currently known to not cause Lethal White when in dominant form. Therefore, a horse can carry two copies of W20/be homozygous for W20.
W22 - Descendants of Not Quite White (1989 Mare). "W22 occurs on the W20 background, that means that all horses with the W22 mutation also have the W20 mutation. Since the W22 mutation has a greater impact on protein function than W20, the reported allele is W22." -UC Davis. Meaning, you may record it as W20/W22 (compound heterozygotes - two mutant alleles on the same genetic locus) or W22 in your horses reference information (the W20 will be understood). *I do not know for sure if this information is also true for other W mutations past 20 (W25, W26, W27), though I would assume it is true for them, as well - if anyone can verify this information for me, please comment below with links!*
W22 can cause sabino-like white spotting (as seen in Airdrie Apache, Silver Cindio, Color Notch Up, Indy Kitty, and All Decorated), sabino-like roaning (as seen in Airdria Lace and Aladin Dancer), near-white coats (as seen in Not Quite White , Arctic Mystique , Arctic Spirit, Lake Barkley, Art in Motion, and Polar Foxx [close up]), or it can cause pure white coats (as seen in Arctic Cielo and Arctic North).
    Side Note: It's interesting to note that Silver Cindio and Aladin Dancer are full siblings, and yet have different expressions (both have sabino-like spotting, but only one has false roaning).
W25 - Descendants of LaughYouMay (2001 Mare). She is pictured with her Dominant White + Frame Overo colt. So far W25 Can cause sabino-like white spotting, and near-white coats on it's own. 
W26 - Descendants of Marbrowell (1997 Mare). On its' own, W26 can cause sabino-like white spotting, or it can cause near white coats (as seen in Mellow Yellow and Happy). However, when paired with Frame Overo, it can cause a pure white coat (as seen in Maybe Mabelline who is dom white + frame overo).
W27 - Descendants of Milady Fair (1960 Mare) pictured with her filly Glacial. W27 causes sabino-like white spotting to near-white or pure white. Milady Fair produced a pure white filly in 1966 named Glacial. You can see a colored photograph of Glacial here. Glacial produced a white coat, Khaleben who went on to produce three white foals, including the stallion Colorful Gambler who is the most well known in this family line.

Old HARPG White Spotted and/or Splash Starters
2024 - I am starting a small side project to publicly display the converted family lines. In other words, if an old family line has been designated by TS as a specific W gene (or swapped from nSb to nSpl), we will display it here to make the conversion of descendants from nSb to nW easier. The genes were chosen based on the design of the horse, as well as its descendants designs. 
Only starter horses (without HARPG pedigree, without working link to HARPG pedigree, or with IRL pedigree) or 1st generation horses with one starter parent (or unlinkable or IRL parent) that were posted before 2015 qualify to be recorded. 
Disclaimer: The recording of names for this database are in no way a denotation of the horses registration with TS - it is simply an informative database for the descendants of these horses to reference as a heads up for what to expect when registering relatives to these horses. 
Names are recorded in alphabetical order (disregarding stable prefixes):
 RD's Devil Wears Prada - nW5
 Ljungeld - nSpl/nW5/nW20
 Maximillion - nSpl
 Sumerian - nW5
 Veridical - nSpl/nW5

Other Markings and Colors

Bend-Or Spots
Also called Smut or Grease Spots, these spots are non-related to the Appaloosa gene, and range from a slightly darker color than the horses base coat, to almost black. The size of the spots range from coin-size to rather large, noticeable marks. Though found in many breeds today, the gene originated from a Thoroughbred named Bend Or, who is thought to be an ancestor of Man O' War, who also displayed this marking.
nBor This horse has the possibility to produce offspring with Bend-Or Spots, or not
BorBor This horse will ALWAYS produce offspring with Bend-Or Spots
Bend-Or Spots are also nicknamed Smut Spots and Grease Spots.

Brindle
Brindle does not have a genetic code for the marking because it is actually a form of Sooty that is mutated to look like little broken vertical stripes.
So, if your horse is Brindle, please include Sooty genes into it's information, either through nSty or StySty.
According to Dr. Philip Sponenberg, "brindle seems to require sooty black counter shading for its expression, and reorganizes sootiness into vertical stripes instead of a more uniform sprinkling of hairs."

Corn Spots
Also called Corn Marks, these are small patches of hair darker than the base coat that are the result of solid-colored hair growing over cuts in the horses skin. Therefore, Corn Spots are not genetic, however they do effect the color of the horses fur in the effected area. On solid colored horses, these spots are almost unrecognizable, however they seem to stand out most on roans, where the dark undercoat shows through the light roan area.

Chimera
Chimera is not a gene on it's own like Splash or Sooty or any other gene is. It is actually two whole sets of complete genes! Chimera is caused when twin foals merge together early on in their dams pregnancy, resulting in a foal with a mixture of two different coat colors. The resulting coat colors must be colors that are genetically possible by the pairing of the sire and dam.
For example, if you breed a Black (Ee/aa) and a Chestnut (ee/aa) horse together, it is possible that the Chimera could be a Black and Chestnut result. Its genetics would therefore be Ee/aa AND ee/aa.
If you breed a Chimera, there is a chance that the foal will take the genetics from either one or the other set of genetics (in this case it will take from either the Black or the Chestnut aspect of the chimera).
Because Chimera is the result of twins fusing together, it is not a passable trait to future offspring, and happens by chance.

Gulastra Plume
This is a tail marking where white hairs mix in with the base colored hairs of the tail. The base of the tail (around the horses dock/where the tail hair starts) is it's normal base color, with the middle and tip of the tail being lighter. 
The marking got its name from the Arabian founder Gulastra, who did not display the tail marking himself, but did cause it to occur in his foals and grand foals, etc (also notice his loud front left leg white and bold blaze). As Arabians helped found the Thoroughbred breed, this marking sometimes pops up in modern Tbs.
The current cause is unknown, but as research unfolds we will update our info! If you find any updated info, please feel free to share links in a comment!
My current personal theory is that it may be caused by a minimal unidentified W gene, as Tbs with Gulastra Plumes often also display interesting face and/or leg white.
Some irl examples include: Big Buzz (face), Helyna's Dreaming (face), Preparense (face), Toho Ullman (face) 

Reverse Brindle
Reverse Brindle does not have a genetic code for the marking because it is actually caused by one of two factors. Most often it will be a form of Rabicano that is mutated to look like little broken vertical stripes. More rarely, the horse is a Chimera with unusual white streaks or swirls.
So, if your horse is Reverse Brindle, please include Chimeric (two sets of genes, one is the base color, one is the lighter color- For example, White Spotting, Cremello, etc) or Rabicano genes into it's information.
When Reverse Brindle is the outcome of mutated Rabicano, it IS possible for that horse to produce true roan offspring, as occurred in the irl Thoroughbred Catch A Bird, who founded the roan color for Tbs.

Somatic
Somatic does not have a genetic code for the marking because it is actually the partial "turning off" of a gene. 
For example, a Black horse with Somatic may have Chestnut areas showing because the "E" gene was turned off in a concentrated area. A Bay horse with Somatic may have Black areas showing because the "A" gene was turned off in a concentrated area. Likewise, a Buckskin horse with Somatic may have Bay areas showing because the "Cr" gene was turned off in a concentrated area. A Chestnut with Somatic may have gray areas where the chestnut color did not pigment correctly. 
Somatic differs from Chimeras because the area(s) effected will be concentrated to generally one or two areas, big or small, though generally small- not scattered about liberally. Chimeras also have two gene sets whereas Somatics just have one like a regular horse. The most common examples of Somatics are: Fleabitten Grays, Grays with Bloody Marks and Mulberry Grays. The Gray gene is partially turned off in these instances, causing the base coat to show through. 

Vitiligo
Vitiligo does not have a genetic code for the marking because the current theory for the marking is that it is actually a side effect of an auto-immune problem that has an unknown cause, it's not particularly a marking gene. The most common area affected is around the eyes. The effected areas are left depigmented, and are susceptible to sunburns.