Preheat oven to 350°F and grease a 9x13 baking dish.

In a large mixing bowl, beat cake mix, water, egg whites, oil, sour cream, and vanilla until combined. Mix on low for 30 seconds, then on medium for 2 minutes.

Pour 2 cups of batter into prepared pan.

Divide remaining batter into three bowls. Add a few drops of blue, pink, and yellow food coloring to each, stirring well.

Drop spoonfuls of each colored batter randomly on top of white batter. Swirl with a knife.

Bake for 30-35 minutes until a toothpick inserted comes out clean. Cool before frosting.