Heat oven to 350°F (if using dark or nonstick pans, heat oven to 325°F). Spray bottom only of 13x9-inch pan with cooking spray.In large bowl, beat cake mix, water, oil, egg whites and 1/8 teaspoon paste icing color with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour batter into pan. Bake 26 to 31 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.