Bake two 1-layer cakes; cool 1 top side down, 1 top side up. Ice cakes smooth in buttercream. Pipe tip 5 ball borders in assorted colors. Position color flow pieces.
Note: Buttercream icing will break down color flow. Position color flow pieces on a piece of parchment paper or waxed paper cut to fit, on sugar cubes or on mini marshmallows.