We haven't done this in months and the kids were pretty excited when I told them I had actually remembered to pick up shaving cream while shopping (unfortunately I accidentally forgot to get a non-scented kind so my kids smelled all menthol-y and fresh for a little bit after their bath).
Here's the low down.
1. Buy the biggest, cheapest can of shaving cream that you can find.
2. Fill several small plastic containers with loads of said shaving cream.
3. Squeeze a couple of drops of food coloring into each container of shaving cream.
4. Carefully stir until the color is evenly spread.
5. Hand the "paints" over to your children in the bathtub and watch fun happen.
The nice part is that the shaving cream rinses off easily (we just detach the shower head and spray around the bathtub enclosure which does the trick beautifully). Also, the "paints" are quick and easy to throw together and they don't stain your children which is nice.
Hip hip hooray for paint baths!