Divide frosting into 3 small bowls (1 cup/250 mL each). Stir 1/2 teaspoon (2 mL) different colour of food colour into each bowl. Fit 3 separate decorating bags with 1/2-inch (2.5 cm) round piping tip; fill each with different color frosting. Place first cake layer on plate. Using green frosting, pipe around top edge of cake layer, forming a thick frosting border. Spread additional frosting over top of cake inside frosting border. Top with second cake layer; repeat border and filling with purple frosting. Place third cake layer on top; repeat with orange frosting. Pipe remaining frosting into mounds over top of cake, alternating colours and sizes of mounds.