The kicker is that I use a Mac Pro and would have to run SAI within a virtual machine if I want to use it. However, I shouldn't have to since I have Manga Studio 5. The problem is that there isn't a great deal of information about Manga Studio 5 in relation to Photoshop or SAI or any other paint/drawing program. The information I did locate wasn't very helpful either.
So, I played around with Manga Studio 5 this morning to see if I could locate a rough equivalent to the Shade and Luminosity layer modes that are found in SAI. I think I figured it out.
Shade Mode: The equivalent layer mode I could find in Manga Studio 5 for Shade was Color Burn. I reduced the opacity of the layer to around 60% and used a grayish purple color to paint my shadows. The results were very similar to that of the Shade layer mode in SAI, using a similar shadow color at around 30% or so.
Luminosity Mode: The closest I could get in Manga Studio 5 to SAI's Luminosity layer mode was the Add layer mode. I tried Color Dodge, but it didn't appear to work as well as Add. (TBH, the jury is still out on this one.) The one downfall I can find is that you might have to highlight different areas on separate layers so you can adjust the opacity of the layer… OR… you can adjust the opacity of the brush to accommodate different areas of your work. What I found was that lighter areas required less opacity (like around 30-40%), whereas darker color areas required more opacity (like around 40-50%) in order to accomplish the effect that I was looking for.
I think that Smith-Micro should consider adding a Liner Dodge layer mode to Manga Studio 5. I'm not exactly sure why it wasn't included as neither Color Dodge or Add work exactly the way I was hoping for… but they are a close substitute, I guess.