I got the idea and tutorial here. Omnomicon makes her cake low-fat and low-calorie, but you can make the very same cake using traditional directions with egg and oil and will turn out exactly the same. I did follow her directions using the soda pop- mostly because I didn’t realize that was optional until after I made it- but the cake turned out wonderful anyway. I didn’t eat it (gluten), but the adults who did said it was moist and crumbly and delicious, and it baked up beautifully.
Hairyshoefairy has made it too- first try and second try, using her own technique and getting interesting results. See? You can totally have fun with this cake!
In a nutshell, use two boxes of WHITE cake mix, mix it up as you prefer, and then divide it equally between six bowls. Use food color to dye them as bright or as pastel as you prefer. They will look exactly the same after baking as they do in your bowls, so don’t worry about them changing during cooking.
Grease your pans- I used cooking spray with flour in Calphalon 9″ cake pans, and had no problem at all with sticking. Start with one heaping cup of your base color (this will be most of the batter, leaving about 1/3 cup in the bowl) in the center bottom of your pan. In one pan, start your rainbow in order: Red, Orange, Yellow. In the other pan, do the opposite order: Purple, Blue, Green using the heaping one-cup. Then to finish, make the rest of the rainbow using the remaining 1/3+ cup of each batter to finish each pan. They’ll look like this, opposites of each other:
I left them carefully layered, but as you can see in HSF’s links, she got all crazy with mixing them, and it turned out just as pretty. So remember this isn’t rocket-science and have fun! I skipped Omnomicon’s lowfat frosting and just make regular buttercream, and it worked great. It was definitely a hit, and I’ll make rainbow cake again.