I think most little children love to paint, and because they are uninhibited by fear of failure, they often create beautiful artwork even when they just splash paint onto paper. We can easily channel this fun activity into an educational opportunity.
For instance, while a toddler is painting (I recommend letting them use the little Crayola watercolor boxes) say the names of the colors he is using and have him repeat them.
For an older child who already knows the color names, show how mixing 2 colors together produces a third color.
Explain what makes primary colors primary: they are colors that you cannot make by combining other colors but can combine to make any color other than white, black, or grey.
Help your child make a sunset scene, like the photo at the top of this post, by gradually mixing red and yellow (click the link for the instructions for creating the scene).
For the more advanced, I think you could use red and blue and white to create a sunrise scene similar to the sunset scene.
Let go of your inhibitions and have fun painting with your child. You just might learn something, too.
artwork courtesy of Ideas for Children's Art Lessons