Divide frosting among 3 small microwavable bowls. Microwave 1 bowl on High about 5 to 10 seconds; add few drops yellow food color and stir until frosting is smooth enough to drizzle. With spoon, drizzle yellow frosting back and forth over cake in striping pattern until all frosting is used (allow some frosting to drip down onto plate in middle of cake, if desired). Microwave second bowl of frosting; add red and blue food color to make purple. Drizzle frosting over cake in same striping pattern. Repeat with last bowl, adding red food color to make pink frosting. Decorate cake with sprinkles.