The recipe is:
2 tbsp cornstarch
4-6 pumps baby shampoo (about 1/8 c.)
2-3 drops food coloring
1-2 tsp water
It wipes off real easy with water. I wiped this paint job clean with the shower and a luffa. NOTE: You can't paint on a wet surface so make sure the tub walls are dry before painting.
Look it up also on my Pinterest: http://pinterest.com/shaydopp/kids-ideas/