Another wonderful book for preschoolers about exploring colors and color combinations is Mouse Paint by Ellen Stoll Walsh. This simple experiment is a fun follow-up to the book.
Six clear containers
Red, yellow, and blue food coloring
1) Mix food coloring and water for each primary color.
2) Using an eye dropper, transfer color from one container to another. Mix to make secondary colors.
Afterwards, I let the girls experiment and see what colors they could make!