gness7 said: The regular ol' water-based food coloring you get from the baking goods aisle of the grocery store should be good enough and won't stain anything.
I used to use food coloring which would stain if you use enough of it to get a decent color out of it. I switched to using body paint if I need a liquid or cosmetic pigment if I want a powder. A lot of people use paint and other things you should not be putting on your skin. Non-toxic paint is still not something you want to be covered in IMO.