Many thanks to Kimberly Stoney who sent me this color mixing idea and recipe! It worked wonderfully. Kimberly tests and develops crafts for a children's magazine. Doesn't that sound like a fun job?!
We had tried mixing two different colors of tempera paint in a plastic bag after reading about Gwyn's color mixing project on My Kids Art, but the paint was so intense that we didn't get very good results. The orange we mixed was way too red and the purple was also way too red. Maybe we just used too much red... But when Kimberly e-mailed me the recipe for Rainbow in a Bag, I was excited to try it. Here's the recipe she sent:
Rainbow In A Bag Combine 1 cup cornstarch, 1/3-cup sugar, and 4 cups cold water in a large pot. Heat and stir constantly. Once the mixture begins to thicken, remove it from the heat and continue to stir for another minute. Divide the mixture into 2 bowls and set aside to cool.  Add a few drops of food coloring to each bowl (one red bowl and one yellow bowl for example). Put some of each color mixture into a Ziploc gallon bag (and seal with duct tape if necessary). Let the child mush and mix the two colors together. Hang in the window to let the light shine through.
My notes: I divided our mixture into three bowls for red, yellow, and blue, so that we could try mixing orange, green, and purple. We used sandwich bags and didn't need the duct tape.
This recipe is easy to make (I was just starting to get over the flu when we did ours a few days ago so easy was a prerequisite), and creates a whitish base that you add food coloring too. I think the base is what helps make it effective as a color mixing project.
