I saw this shared on Facebook a while ago and i thought it was a great idea. A great messy activity for in the bath. All you need in shaving foam and food colouring then you mix a bit of food colouring in with shaving foam and there, you have your bath paints.
I put ours in silicone cake cases as i thought it would be easier if it dropped in the bath and for them to hold. I gave them a brush and they was happy until the water went cold. Then it just wipes off your tiles with a sponge.