Preheat the oven to 350°F and spray a 9x13-inch baking pan with baking spray and set it aside.
Add the cake mix, water, oil, and egg whites to a large mixing bowl. Using an electric hand mixer, mix ingredients together on low speed for 30 seconds. Increase the mixing speed to medium and mix for 2 minutes.
Put ¾ cup of the batter into a small mixing bowl and repeat using another bowl.
Add blue food coloring to one of the bowls until you reach your desired shade of blue. Repeat the same thing with the second small bowl using red food coloring this time.
Carefully, dollop each of the colored batters into the prepared baking dish. Evenly distribute the colors throughout the whole cake, be sure not to put too much of one color next to each other.
Use a toothpick to swirl the colors together.
Bake the cake for 29-34 minutes, or until a toothpick can be inserted into the center and comes out clean.
Remove the cake from the oven and let it cool completely in the pan on top of a wire rack.
Once the cake is completely cool, add the whipped topping and garnish with berries if desired before slicing and serving.