Changing the color of a Sheep’s wool is a vanilla MC thing you can already do with dye items, but not with GT Spray Paint cans as far as I can tell. Since you can already use spray paint to change the color of wool blocks in the world, it would make sense (and be fun) to be able to use spray paint on the sheep itself as well.
If memory serves, this does have the side effect of making all wool the sheep produce in the future that color, and that color can also be passed on to its offspring. And sure, I get that that’s not exactly realistic, but since it’s already a vanilla Minecraft thing that shouldn’t be too game-breaking.
I remembered that tamed Wolves and Cats have a similar mechanic where you can right-click them with dye item to change the color of their collar. That might be something spray paint could be used for as well.