2 c water
1/4 c cornstarch
food coloring
Dissolve corn starch in a little of the water and set aside. Bring remaining water to a boil, then add cornstarch slurry. Remove from heat (divide, if making multiple colors), then stir in food coloring and allow to return to room temp before using. Paints will thicken as they cool.
Time to make: ~10 minutes
Source: Pamela Peterson