1. Place 1/4 cup buttercream in each of 3 bowls; tint buttercream in one bowl red, another yellow, and another blue. Transfer each color to a pastry bag fitted with a coupler and a small plain round tip (such as Ateco #4); set aside.

2. Using a serrated knife, trim tops of cakes level. Place 1 cake layer on a cake stand or a cardboard round wrapped in parchment paper. Spread with 1 1/2 cups buttercream. Top with remaining layer. Thinly coat top and sides with buttercream. Refrigerate 30 minutes. Remove from refrigerator, and frost with 2 cups buttercream. Using a toothpick, make an outline of dashed arches on side of cake. Pipe red, yellow, and blue buttercream dashes over the outlines. Switch to a 1/4-inch plain round tip (such as Ateco #12) to make "balls" of frosting at end of each dashed line. Pipe 6 "balls" around top edge of cake. Cake can be refrigerated up to 6 hours.