"Rainbow Cupcake Frosting" Could be done on rainbow cupcakes. This looks yummy as well as pretty. I'm not a big fan of sweets, and I have little to no fondness for chocolate cupcakes, but I totally want to eat that.
pinner says "Rainbow Swirl: paint food coloring inside bag, then fill with icing. It will "stripe" the icing as it squeezes out - try it in any color combo!" (not sure if this will work with my luck it will turn out all brown and icky)