I added several drops of red food coloring to the tub as it filled, turning it pink. Then I sprinkled the sprinkles on top. I wasn't certain what the effect would be. Turns out about half sank through the bubbles immediately, but others slowly melted into the bubbles, staining them with color.
I added more sprinkles into the cups and teapot, so the girls could play with those, too. When I opened the door to the bath the girls were so excited, they couldn't peel off their clothes fast enough.
After a while the bubbles disappeared, as they do, leaving behind the perfectly pink water, and cleaner and happier girls. Yay!