Preheat oven to 350 degrees. Coat 8-inch baking pans with cooking spray or grease and flour the pans, tapping out excess flour. Set the prepared pans aside.
Using an electric mixer with a paddle attachment, beat the cake mix, pudding, mix, water, oil and eggs on medium speed for 2-3 minutes or until well blended. Scrape the sides of the bowl and mix again until all the ingredients are incorporated.
Pour equal amounts of batter into 6 cereal sized bowls. Using Wilton Color Right system, color each bowl a color of the rainbow.
Pour batter into prepared pans, filling each approximately 3/4 full.
Place pans in the preheated oven. Bake cakes for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Depending on the size of the oven, you may need to bake this cake in two batches.
Remove the pans from the oven and place on wire cooling racks for 25-30 minutes. Run a knife around the edges of the cakes, flip the pans over and gently extract the cakes. Return the cakes to the wire racks and finish cooling completely.
While the cakes are cooling, make a batch of buttercream frosting. Reserve 1/2 of the frosting into a separate bowl for filling and crumb coat. Separate remaining frosting into 6 bowls. Using Wilton Color Right System, color each bowl of frosting a color of the rainbow (red, orange, yellow, green, blue, purple).