Just mix up a regular white cake mix.
Divide up the cake mix into several bowls. Use food coloring to create the desired colors. I just mixed up the basics: Red, yellow, blue, purple, orange, and green.
This is where kids love to help! My nephew was a regular artist. Just take spoonfuls of colored cake batter and start plopping them into a cake pan, taking turns to create color patterns. (I used a bundt pan this time, but it works just as well in a rectangular pan too!). I remember as kids, trying to make sure that we didn't let the same colors touch.
Bake according to the directions. I realize I could have decorated this MUCH cuter (how about using green-colored coconut for grass??), but this was an impromptu idea. So we just went for good ol' yellow frosting.