1Heat 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.
2Bake 25 to 30 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
3In small microwavable bowl, microwave white chocolate and whipping cream uncovered on High 1 minute, stirring once, until softened and chocolate can be stirred smooth. Cool 10 minutes.
4In medium bowl, beat butter and 2 cups of the powdered sugar with electric mixer on medium speed until blended. Add white chocolate mixture; blend well. Add remaining 1 cup powdered sugar; beat until smooth. Add orange food color and blend well. Spread frosting on top of cake.