These little goodies need very little explanation. Just take a white cake mix, divide up the batter (5 or 6 bowls), add food coloring (the good kind that makes nice, rich colors) and spread a little layers of each color into each muffin liner (I used white liners so that you could see the rainbows through the liner).
(the tricky part is having enough of each color to make the six layers- I would use a regular box mix to make 18 cupcakes instead of 24, that way you have enough of each color with a tiny bit left over). I have also tried this to make "candy corn" looking cupcakes with yellow, orange and white, and they looked really cute as well (making three layers was even easier).
Lexi showing off her rainbow cupcake.
Grace loves rainbows (coloring them, finding them when it rains, and now eating them!).