PREPARE cake batter and bake as directed on package for 13x9-inch pan, lining pan with foil sprayed with cooking spray before adding batter. Cool cake in pan 15 min.; invert onto wire rack. Cool completely; cut into pieces as shown in diagram. Discard cake trimmings or reserve for snacking. Arrange cake pieces on platter as shown in photo to resemble guitar.
TINT 1 cup COOL WHIP with blue food coloring; spread onto top and sides of neck of guitar. Tint remaining COOL WHIP with yellow food coloring; spread onto remaining cake.
CUT 1 licorice string into 2-inch lengths. Arrange 6 pieces on neck of guitar for the frets. Decorate with all remaining ingredients as shown in photo. Keep refrigerated.