3 egg whites
1 (18.25 ounce) package white cake mix
1 1/3 cups water
2 tablespoons canola oil
red paste food coloring
orange paste food coloring
yellow paste food coloring
green paste food coloring
blue paste food coloring
purple paste food coloring

1. Preheat an oven to 350 degrees F (175 degrees C). Grease and flour two 8-inch round cake pans.
2. Beat the egg whites with an electric mixer until frothy, about 1 minute. Add the cake mix, water, and canola oil; continue beating for 2 minutes on medium speed.
3. Divide the cake batter into six separate bowls. Use a toothpick to scoop a dab of food coloring into one bowl of batter and stir; add more food coloring, if necessary, to reach the desired shade. Repeat with the remaining colors and bowls of batter.
4. Scoop spoonfuls of batter into the prepared pans, alternating the colors. Use a toothpick to gently swirl the colors for a marbled effect.
5. Bake in the preheated oven until a toothpick inserted into the cake comes clean, about 30-35 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.