First, the girls sprayed shaving cream into muffin cups (and really, haven't you had fun right there?). Then they dropped in a few dots of food coloring and swirled:
Don't these look delicious? ;)
When they were done, they had a lovely rainbow assortment of bath paints. We happened to have a set of neon food colors in addition to the usual ones--and we experimented by mixing other colors--so we made a variety of shades.
The girls used regular paint brushes and sponge brushes to paint our bathtub walls, etc. They had a blast! And although I don't have any pictures of the paints actually being used (way too many naked tushes :), I think this "after" picture gives a nice feel for how much fun the girls had: