Heat oven to 325ºF. Generously spray a bundt cake pan with cooking spray.
Make cake mix according to the box directions, using water, oil and egg whites.
Pour 1 1/4 cup of the batter into a small bowl and color it red with food coloring.
Pour 1 cup of batter into another bowl and mix in blue coloring.
Pour red batter into the bottom of the prepared pan. Carefully pour white batter over red batter in pan.
Now, carefully pour blue batter over white - it doesn't need to cover the white all the way.
Bake as directed on box, or until toothpick comes out clean. Cool until warm and them flip over onto a serving platter or plate.
When cake is cool, combine powdered sugar, milk and vanilla in medium bowl and mix until fully incorporated. I like to add the milk one tablespoon at a time to make sure it doesn't get too runny. If glaze isn't runny enough, add more milk and if it's too runny, add more powdered sugar.
Drizzle over the top of the cake. Slice and serve.