If I'm reading this correctly, which is the fact that the robes don't have skins on specific models, then I'll break it down for you;
Skins are handled by a basic line of code within the models file that tells the game what its switching between, aka a texturegroup. This texture group consists of the materials that are responsible for changing the color of the facial hair color, the hair color, and anything else deemed in between. The problem with this method is that there can only be one texturegroup at a time that handles specific material changes, due to source limitations. The only work around to this is adding near 20+ skins manually that have all variations of robe and hair textures at a time.
Put simply, having the option to change the hair skin overrides the ability to change the robe skin. JJ had a clever work around for this by making it so that bounty hunters can change their armor color via the player selector compared to skins, that way the skins can be left to change the hair color. So for now, there is no fix to this without either removing dynamic hair or removing dynamic robes.