I saw this idea on No Time for Flash Cards.
We wrote on a piece of paper the primary colors added to each other, and our experiment was to discover what colors they made. We have paint in the primary colors. So I squirted some paint onto 3 paper plates and had the boys both mix the paints with paint brushes. Then I had them paint with the new colors on a piece of paper.
One by one, we filled in the answer on the paper for what the new colors were.
LM especially loved this and wanted to do it again and again. :)