Mix vinegar and baking soda in small bowl. Slowly add cornstarch and glycerin.
Pour one teaspoon into each paper cup. Let dry overnight.
After they've dried, add food color. The color isn't as deep when it dries, so remember to add a lot of food color.
Dip a small paint brush into some water, and then into your paint. Use good construction paper that will soak up some of the water.