Ok, this is stupid easy but I thought I'd share in case anyone else might like it. Sometimes we like to bake a cake, for no special reason other than that cake sounds good. I always let the boys decide how they'd like it decorated, and yesterday was their second request for a "rainbow" cake. There may have been others who've done this before, but I came to this technique on my own.
Step 1: Bake a cake using the boxed cake mix of your choice.
Step 2: Frost cake with canned icing of your choice.
Step 3: Put a few rows of dots of red, yellow, and blue food coloring.
Step 4: Starting in one corner, smear a knife or cake frosting spatula thingie back and forth across the cake, smearing and blending the colors until it looks nice and rainbow-y. Don't overdo the smearing and blending or your cake will just be brownish grey. Less is more here.
And that's it!
BONUS: Here are two other "just for the heck of it" cakes we've made. Mario mushroom cake and Earth cake!