Easiest recipe ever! I got this from Crafty Soccer Mom.
1/3c dish soap or baby shampoo
Mix the soap and cornstarch. Put a little in 3-4 small containers to make various colors. Mix in food coloring and ta-da!
The girls had a great time with this and the paint comes right off with water (obviously that's the point :-p). Since one of the ingredients is soap we ended up with an unexpected bubble bath as well. :)