First, prepare the cake batter according to package directions. Divide the batter among 6 glasses or small bowls. Mix each of the colors of the rainbow- red, orange, yellow, green, blue, purple (unless you are super good with food color mixing and want to try for indigo and violet).
Next, using a small spoon, put one spoonful of each color in order into the center of a paper lined muffin tin cup. I tried this with 1/2 of them lined and 1/2 of them unlined and the unlined ones did not rise properly for some reason. The colors will layer very pretty if you just put each color right on top of the next, without worrying about getting a solid layer all the way across.
Bake at 350 degrees until a toothpick inserted into the center of the cupcake comes out clean.
Top with icing and decorate as desired! Laura added a rainbow Lucky Charms marshmallow on top of this one. I thought it was a cute touch, especially for a 3 year old to come up with!