You will need
:: one ice cube tray
:: red, yellow & blue food coloring
:: 1-2 droppers
Fill an ice cube tray with H2O.
Add a drop of food coloring (primary colors only) to either 3 or 6 of the compartments.
(I did 2 drops so the yellow was more orange.)
Demonstrate how to add a drop of two colors to the water to create a new color.
Watch your child discover the theory of color!