Just looking for ideas here. My initial thought was wave property of light, just using a pinhole image and a water analog, but still thinking of ideas.
Why Poop is Brown
by Vera Lynn
My poop is brown, even when I don't eat brown food. When I mix my finger paints all together, they turn brown.
I think my poop is brown, because my food gets mixed up inside me.
Experiment: Have finger paints of different colors available, and mix them, showing different resulting colors. In then end, have the children mix all the colors together.