Red, orange, yellow, green, blue and purple gel food coloring
Preheat the oven to 350°.
Grease, and then flour two 8" round cake pans.
In a large bowl, with an electric mixer, beat the egg whites for about 1 minute until they are frothy.
Add the cake mix, water and oil to the large bowl and beat for 2 more minutes on a medium speed.
Divide the cake batter into six smaller bowls. There should be one bowl for every color of food coloring.
Add a dab of red food coloring to the first bowl of cake batter and stir. Add a dab more until you get the desired color of red.
Repeat with the rest of the bowls to create orange, yellow, green, blue and purple batter.
Using a large spoon, scoop separate batter colors into the round cake pans. Make sure to alternate colors and do not mix the batters together. If you'd like, you can use a toothpick to swirl the colors together to create a marbled effect.
Bake for 30-35 minutes until an inserted toothpick comes out clean. Cool for 10 minutes before transferring to a cooling rack.