Ok FMA art and style is really frustrating in the fact that theirs always a freaking exception! Any rule you make, any! Its always like "but oh! So and so has..." yea-no. Is is to much to ask for a little conformity here? Realy!?
Ok that rant put aside...
THERE ARE SOME EXEPTIONS TO ALMOST EVERY "RULE" I PUT DOWN HERE
So technically yes, you could give your OC bright purple hair with a pink fringe, but bear in mind that that will look very out of place in the FMA world. So there's just one thing for you to bear in mind when breaking these rules: Subtlety. The broken rule should fit in with the rest of the FMA world and not stand out like "holy cow where are they from! It should not be the first thing you notice when seeing the character, nor should there be two broken rules in the same OC. Ready then? So lets begin
This is one of the few absolute rules DO NOT GIVE YOUR OC AN EYE COLO