All you need for this is a piece of paper, paints (preferably washable tempera) in all the colors of the rainbow, and Qtips.
Explain to your child what you'll be doing. If your child will it exactly how it's supposed to be, great! If not, here's a helpful trick. Cover the sections of paper underneath the part that's being worked on with another sheet of paper, moving it down as needed. This won't stop them from adding colors where they don't belong, but it will help. The point is to use the Qtips to make dots of each color on the page in sections that are about the same size, in the right order of course. Ours turned out beautifully!