Rainbow Layer Cake recipe from Betty Crocker. 2 boxes vanilla cake mix divided and colored with gel food coloring. Idea from comments: Use regular whipped canned frosting for middle layers and Cool Whip refrigerated frosting for the outside.
Pastel layer cake with Swiss meringue buttercream (Martha Stewart. Five pastel layers form this colorful creation. You can also use the batter to make four dozen cupcakes or a two- or three-layer cake.