Might be it's a bit difficult with the current system.
It's not just a color options you see, it's a layer, it's has to be made first before adding into the game. The game isn't that system so the creator has to make a assets to do it and it's also probably have to do with the current system, manually having to added a section in as well as making it not cause problems with others clothing so it's work. Though that's my guess.