As it happens, these kids’ roller ball bath paints:
…don’t show up on black skin. You just have to make a really big deal about how your kid has green skin when in reality, it is just slick looking black skin. Fail. Unless you’re S. who really doesn’t notice because all he wants is an excuse to hear you fake scream with horror over and over that he is turning into a frog. So win on his end I suppose.