Ice the giant cupcake. Tint icing following formulas above. Prepare four decorating bags with tip 45; fill with tinted icing and white icing. With tip in the horizontal position, smooth edge facing upwards, pipe vertical lines from top to bottom on cake, alternating colors.
Place crisp rice cereal treat top on top of cake, attaching with small amount of icing. Spatula ice smooth with white icing.