My kids were amazed! They painted everything in sight. I brought in the play watering can so they could “melt” their artwork off the tub.
Some other helpful tips from the post comments and our own experience:
We used a 6 pack of these Fizzy Tub Colors dissolved in a few tablespoons of water, then mixed with the shaving cream. Also tested washable tempura paint (sparingly!) and it washed off the grout with a good washcloth. Don’t use food coloring if you don’t want to scrub your tub.
PS shaving cream is NOT tear-free in case your boys start ~~painting each other like clowns~~ a paint fight.