I made a rainbow cake for my brother for his birthday and it turned out pretty darn cute!
I thought you might want to make one for yourself (or someone you love) so here are the instructions:
Buy one white cake mix and prepare batter as instructed. Once mixed, divide batter evenly into 6 bowls or cups. Using food coloring, add one color per bowl and mix.
To make orange, mix 2 drops red and 2 drops yellow
To make green, mix 2 drops blue and 2 drops yellow
To make purple, mix 2 dropes of blue and 2 dropes of red
Butter and flour your cake pans and then pour the batter in one color at a time in the middle, on top of each other. The colors will thin out in the pan as the next one is poured on top of it, like this:
Bake as appropriate, let cool and then frost.