We've been doing this walking water color wheel experiment for around 13 years. The older girls are over it, but Ada still screams with joy when she checks its progress each morning.
To do this experiment, put 6 short jars in a circle. Fill every other jar about a third full with water. Put blue food coloring in one, yellow in another, and red in the last. Connect each jar to its neighbor with a folded paper towel. The water will immediately start traveling up the paper towel via capillary action, and in a day or 2, your primary colors will mix to form secondary colors and your color wheel is complete!