This does sound like a neat little idea actually. But I would think it would make some characters rather messy. A lot of people just do presets if they want specific markings and more than just two colors.
This would honestly be great to have! It would add a lot more variety in characters, but as others have stated, it would be hard to get this to happen when the staff don't have the right resources to make it happen.
I've thought about it a few times but never thought about bringing it up in a thread~