Heat oven to 350°F. Spray bottom only of 13x9-inch pan with cooking spray. In large bowl, beat cake mix, water, oil and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Divide batter evenly among 4 medium bowls. Add a different food color to each bowl to make orange, red, blue and green. Drop by large spoonfuls into pan side by side, alternating colors.