Martha Stewart's Rainbow Cake is an impressive cake. The ingredients are vegetable shortening, 4 cups of all purpose flour, 4 teaspoons baking powder, 1/2 teaspoon salt, 2 sticks room temperature unsalted butter, 2 1/3 cups of sugar, 5 large egg whites, 2 teaspoons pure vanilla extract, 1/1/2 cups milk and food coloring in red, orange, yellow, green glue and purple. The icing is Lemony Swiss Meringue Butter Cream. Full directions can be found here: http://www.marthastewart.com/256688/rainbow-cake.
To make a rainbow cake you will need 6 cake pans, 6 different food colors, and cake batter to make 6 thin cake layers. You will also need a lot of vanilla icing and decorations of choice. You can view a video and the finished product here: http://www.youtube.com/watch?v=U5vowzpgmSc.