I recently found online instructions for a rainbow cake (can't remember where, though). I thought it'd be fun for Superman's birthday cake.
I started with a white cake mix and divided the batter into six bowls. I added food coloring to the batter and then mixed it up. My boys loved watching the batter change colors and then telling me what color it was. ROY G BIV into the cake pan...
Superman wanted a green dinosaur on a green background. I tried to talk him into another color, but he was adamant.
This is what it looked like on the inside.
I made rainbow cupcakes with the other half of the cake batter.