For the first step, just copy the recipe for Chocolate Fudge Cake, but without melting Dark Chocolate and without adding Cocoa. I also reccommend not adding Salt, but that bit is optional. Instead of the Melted Mixture, add 150ml of Food Dye (ONLY ONE COLOR). Just don't add Custom Colored Food Dye or it will look "dead". The recipe is below.

The only colors you don't need to use are Black and White. When you finish making all 7 rainbow doughs, you should get something like this:

Make anything your heart desires! Here are some examples: