This is one of my favorite activities about color: Making a color wheel. We sent a note home asking for the children to collect random objects from different colors of the rainbow. This gives such an assortment, plus it's more meaningful since the children brought them from home.
We then applied the paint to the paper. First the primary colors, then we mixed those to make the secondary colors.
Once the paint was dry, the children sorted their objects by color.
They then glued them to the matching color on the wheel.
I originally got this idea from reading Kristin's website. Look at her cool color wheel at Preschool Daze!