This craft is super simple and VERY cheap. All you need is equal parts cornstarch and water, plus food coloring.
My nephew had alot of fun putting the food coloring in the mixed cornstarch and water, we did the 4 primary colors to start with then mixed the different colors when we started running low.

P.S. He LOVED dipping his hands in the "paint" and leaving hand marks on the sidewalk.

BONUS! Cleaning the sidewalk with the hose was just as much fun and making the mess :)