Ok, I know this is obviously to late to help with your dilemma but just for the future.
Using White, Yellow, or Vanilla cake mix add a but of nutmeg, cinnamon, and/or vanilla extract. It's a tiny difference in ingredients but it makes the cake taste SO much better.
just for a really awesome look! Using white or vanilla cake, you can still follow the above tip but with a very small amount so as not to make the cake to much darker. 1) Split the batter into 4 to 6 bowls, try for an equal amount in each. 2) Put food colouring into the bowls, a different colour per bowl. Now here's where it differs from cake to cupcake FOR CUPCAKES: 1 )put the little paper cups (use the foil type if possible so they can't see the effect until they go to bite it!) in the tray 2)layer each colour on top of each other until you reach the height you would normally fill it, it's ok if it looks messy right now! 3) bake according to the box 4) frost however you wish! once someone bites in they'll have an inverted rainbow in their cupcake! FOR CAKE: 1) layer the batter like you would the cupcakes until you reach the level you would normally fill to. 2 )Proceed like you would any other cake. I would use a thick white frosting for this just so they see this clean white cake and then when they cut it open they have this amazing rainbow cake!