Blissfully Domestic has a nice way for kids to understand secondary and tertiary colors. Cook up some cornstarch clay (like homemade Play-Dough), seperate it into three sections and add red, blue, or yellow food coloring to represent the primary colors. Divide each primary color into thirds and mix with one of the other colors! Tada--secondary colors. Continue and mix again with the secondaries to get tertiaries. The original post shows the colors and the display.