1 box white cake mix (add a box of dry vanilla pudding powder to the dry cake mix for extra flavor).  Add the oil, eggs, and water as the box says. 

Divide it into 6 bowls and add food coloring to each, making rainbow colors.

For cupcakes, which is what I made, add about a teaspoon (or less) of each color in rainbow order (red orange yellow green blue purple).  I also did half the opposite way, starting with purple.

Looks great, tastes great!