I'm sure most people have seen this, but its something we used to do with first graders when I was in my El Ed program at BYU. It's an easy way to help kids see how colors mix. Jackson has known his colors for a long time now, but still would get confused between red+yellow= orange, and red+blue= purple. He had yellow+blue= green down pretty good. So I pulled out the milk (2% works better because its whiter than skim), and we added red, blue & yellow food coloring to three of the cups. Then I let him mix them, telling him how. VERY SCARY. Four year old with food color, on his own, because I had my hands full taking pictures, and holding Owen.
Here's Jackson's finished Rainbow. He really remembers it better too. Once he can do it himself, he really understands it better.
And the best part of learning colors, is that mom doesn't want to waste milk, so for the next three days they got to drink their rainbow. Except green. I spilled green on top of everything in my fridge. Yep, that was a blast, and I spent one hour of precious nap time cleaning it all out. Rawr.