Decorate cake. Use blue decorator icing pouch with coupler and tip 10 to outline triangles on top and side of cake. Use white decorator icing pouch with coupler and tip 10 to outline two rows inside blue diamond. Use black decorator icing pouch with coupler and tip 10 to outline and fill in center of white diamond.
Use white, yellow, black and blue decorator icing pouches with couplers and tip 10 to outline additional diamonds outside of blue one following this pattern: two white, one yellow, one white, one black, two white and one blue. Continue piping this pattern until cake is covered.