Add corn starch and water to food processor, add food coloring til you reach desired color. Pulse til fully combined and slightly watery. Pour mixture onto plate with a large lip or a shallow bowl. Let dry out overnight. Once fully dry, break up disk and place back into food processor or coffee grinder. Grind until powdery.
Repeat entire process with as many colors as you would like.
3/4 cup corn starch
1/4 cup water