this cake is not a rainbow inside, but its all about the bright colors on the outside!
start out with your favorite cake recipe, preferably not chocolate, for obvious reasons. some of my favorite bloggers might have some good ones to try. Hope they don't mind me linking them 8)
whatever white cake recipe you love will work....heck if you must you could even use a *shudder* box mix. I make enough batter to make two 8" cakes. I make the first batch, and separate it as evenly as possible into 3 bowls
I use gel food coloring to add the color. i start with yellow, than orange, the red. because I have only 1 pan that is 8" I try to get the first one in the oven asap. Bake according to your recipe, just be sure to shorten the time, mine were done in about 20-30 min. once the third cake is in the oven, I make the second batch of batter, and split it evenly into 3 more bowls. these get colored, purple, blue and green. yes this is time consuming. but the final product is worth it.
once all the layers are baked, choose your frosting. Anything white will work. Again if you are culinarily challenged you may opt for *shudder* canned frosting. I like to make mine with Italian butter cream with a hint of lemon. this will make a very tall cake, but it looks very striking when it is cut. stack the layers and frosting in order. I brush my cake with simple syrup with vanilla. (equal parts water and sugar boiled together and then cooled. ) baking the layers individually can make them come out abit dryer than i like. once the cakes are all stacked up, finish the cake with the white icing. the effect is lovely.
it is as simple as that. and then you can make a cake with a real WOW factor, without having alot of training, just abit of patience. you can make brighter or paler colors. you can really play around and make it your own. decorate simply to really let the cake shine through, or bring the rainbow to the outside of the cake and give a hint of hte treat held inside.