This is a lesson that evolved in our house. The board we made with plain construction paper helped guide my son as he tried to remember what primary colors combine to make secondary ones. After he figured out that the board can be used as a guide it was a cinch!
Gather your materials. You will need pompoms in primary colors, tongs, and construction paper in all the colors of the rainbow, with an extra blue or red . After doing this I realized we should have had them in rainbow order... not sure why we didn't, if you do you will need 2 strips of red.
Start by adding pompoms to the colors. Your child will notice that there are no secondary pompoms, they have to make the color by placing the pompoms of the primary colors that combine to make the secondary color. Help as needed.