I made this cake for my kids' Superhero Art Party and it was a big hit with all of the children. Sidenote: considering I had 20 eager kids tugging at me yelling "I want rainbow, what does rainbow taste like?" my actual photos of the cut cake are terrible. Please forgive me. We return you now to your regularly scheduled program...you can decorate the outside whatever way you like and if the idea of decorating the outside is terrifying, ice it with white frosting and that would be absolutely beautiful. The best part is that the rainbow layers are so ridiculously easy and that's what I'm going to show you how to do today!
It's important to consider the shape and size of your cake...you need 6 layers (one for each color of the rainbow) the bigger the cake, the more cake mix you'll need. No matter the size I would suggest you need a minimum of 2 boxes of cake mix or your layers will be too thin.
So let's start....I made a "paint can" with the rainbow layers so the pan I used was a 6" round resulting in a tall cylinder shaped cake but you can make any size and shape you want. I used 2 boxes of white cake mix made according to the directions and divided it evenly into six bowls. I added gel food coloring to make each layer the appropriate color. Since your cake will most likely not be the same shape and size I won't waste your time telling you how many drops, I frankly I have no idea. I mixed the coloring in the batter until I had bright rich colors.
A basic art lesson for you....you really only need red, yellow and blue food coloring, that will give you the primary colors of the rainbow. To make the secondary colors....
ORANGE = RED + YELLOW
GREEN = BLUE + YELLOW
PURPLE = RED + BLUE
Bake and cool each layer. I had one pan so I had to bake each layer individually but if you have more than one pan of the right size....go right ahead and bake them at the same time. I kept my eye on the first layer while baking and tested the center with a toothpick when it looked close to being finished, when it comes out clean they are done. Cool the layers on a wire rack.
Once cooled, level the tops of the cakes with a serrated knife to make sure they are flat. Flip the purple layer so the leveled top is now at the bottom and place on a board or cake stand. Spread a thin layer of vanilla or other white icing on top and place the cut side of the blue layer ontop of the purple. I used a can of whipped vanilla frosting....oh my god I can't believe I just said that, nobody call Martha and tell her I didn't make my own frosting.
Continue this process until you complete all the layers. The key here is thin icing, don't try this with big thick rich layers of frosting, it won't look as pretty when it's done. This is a terrible shot of the outside...the top is the can of paint, covered in fondant complete with yellow drips and silver painted accents. The bottom was Spiderman meets Pop Art with white cake and vanilla frosting inside.
So if you have seen this rainbow cake method and hesitated to try it, the baking time makes it mildly time consuming but other than that, it's as easy as pie...or easy as cake in this case.