Place the cake on your cake board in the center of a turntable cake stand. Ice the cake with black icing and, using a cake comb, comb the sides of the cake. Place the cake in the refrigerator or freezer until the black icing is hard to the touch, about 3 minutes.
Once the icing is hard, remove the cake from the fridge and place in the center of your turntable.
Using your other colored buttercreams, pipe into the grooves you made in the black icing with the comb. Turn the turntable and gently pipe the colored icing on the cake. This part does not have to be perfect because we will be scraping it off. Alternate colors to create different horizontal stripes on the cake. Using an offset spatula, smooth the colors onto the cake.
Then using a straight edge, scrape the colored icing off the cake revealing the black lines left by the cake comb.
Use the purple icing to pipe a border on the cake. Fill the top of the cake with Halloween candy and decorate with sprinkles.