When cake is cool, divide frosting evenly into 3 microwavable bowls. Microwave 1 bowl of frosting uncovered on High a few seconds until smooth enough to drizzle over cake. With spoon, drizzle all of white frosting back and forth around cake in a striping pattern. Repeat microwaving second bowl of frosting until smooth. Stir in a few drops blue food color until well blended. Drizzle over cake, scattering frosting back and forth. Repeat with remaining bowl of frosting and red food color, making sure red, white and blue frostings can be seen on cake. Let cake stand at room temperature until frosting is set before serving.