1. A rotating turntable makes cake decorating much easier; if you don't have one, use a plate on an inverted bowl. Place one layer on an 8-inch cardboard round; place the round on the turntable, securing it with a dab of buttercream. With a long serrated knife, trim tops off both cake layers to make flat surfaces.
2. Using a small offset spatula, evenly cover the top of the first layer with 1 cup of the pale-yellow or blue buttercream. Spread the buttercream so it extends beyond the edges of the cake.
3. Place the other cake layer, with the cut side down, on top of the buttercream; press gently to make it level. With the small spatula, spread the cake with 1 1/2 cups more of the buttercream, plus any oozing out from between the layers, creating a "crumb coat," a thin layer of frosting that seals in the crumbs. Refrigerate the cake for 15 minutes.
4. Using a large offset spatula, generously coat the chilled cake with 2 1/2 cups of buttercream, frosting the top of the cake first. Hold the spatula at a 45-degree angle against the cake, as shown, and slowly rotate the turntable. If the buttercream appears to be streaky, dip the spatula into hot water, and wipe it dry, then continue spreading the frosting.
5. Use a bench scraper to smooth the sides of the cake. Hold the scraper perpendicular to the cake with one edge resting on the turntable and slowly rotate the turntable. Small areas of the cake may be touched up with the small spatula. Refrigerate the cake for 30 minutes.
6. Cut an 8-inch round from parchment paper. If you are making the yellow cake, fold the round into 16 equal wedges by folding it in half 4 times. Unfold, and place the paper on top of the chilled cake. Using the fold lines as a guide, lightly mark 16 vertical lines around the sides of cake with the bench scraper.