It's not a bad idea but I just don't see it happening
First off, hair is not very wide spread. Not many people are actually gonna use this and some people won't even see any 3d items at all. Toribash needs to work very big gameplay parts rather than something for a certain demographic.
Second, how would you distribute the color through the hair. There isn't a certain way you can put color in hair. It could change as it goes to the tips, make a pattern, have multicolored strands, and so on and so forth. You would need another forge just like the flame forge, except that it's for something much smaller than flames.
If you wanted these colors in your hair, why don't you find out what color they look like combined and just make that color your hair color?