Heat oven to 350°. Grease and flour rectangular pan, 13x9x2 inches. Beat cake mix (dry), water, oil, egg whites and 1 container yogurt in large bowl on low speed 30 seconds. Beat on medium speed 2 minutes. Pour into pan. Bake 35-40 minutes or until cake springs back when touched lightly in center; cool completely. Fold 1 container yogurt and 1 or 2 drops red food color into whipped topping. Spread over top of cake. Garnish with strawberries if desired. Refrigerate any remaining cake.