I found some cups at Hy-Vee and we stacked them up. Of course as the classes came in, they all wanted to play with them and they had a million questions about them. But after explaining how they worked the kids bought right into it!
It is working really well. It fact after one class he said, "should I let them talk a little?" I definitely think this is something that will stay in the art room even after he is finished with his student teaching!
p.s. they don't work very well for color mixing... I was hoping but the colors just are vibrant enough (and I don't have blue).