Another option is coloring with markers. Test on a small area first to be sure it won't eat the foam. This won't cover strong graphics well, but it does work well over lighter schemes. Also, test your "test" area to be sure the color will be fast after it dries (won't come off on your hands).

With paint, it works, but it's easy to add too much weight. Go easy, 4 light coats better than 2 heavy coats.