Last week I did this with my pair and they LOVED it. I mixed a tiny drop of food colouring with shaving foam. You can get own brand shaving form surprising cheap. I mixed them in a baking scilone muffin tray (not like i was going to use it for anything else!!) It floated on the water too. Once the colour is even its ready to go. Add a paintbrush and let the kids paint themselves, each other and the bath. Then when all done just rinse down the drain.
Their favourite was putting purple prickes on each others backs.
So much fun give it a try.