1Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 13x9-inch pan. Make cake batter as directed on box. Pour into pan. Randomly place 6 drops of food colors over batter. Cut through batter with knife several times for marbled design.
2Bake as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
3Spread frosting over cake. Sprinkle colored sugars on frosting to make rainbow design.