Rainbow Layer Cake recipe from Betty Crocker. 2 boxes vanilla cake mix divided and colored with gel food coloring. Idea from comments: Use regular whipped canned frosting for middle layers and Cool Whip refrigerated frosting for the outside.
"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.