Preheat the oven to 325 degrees Fahrenheit. Spray three 8-inch round pans with cooking spray and line the bottom of the pans with rounds of parchment paper.
In a large bowl, mix the cake mix, flour, sugar and salt. Add the water, oil, vanilla, sour cream and egg whites and using an electric mixer beat to combine.
Divide the batter into 4 smaller mixing bowls. Color three of the batters with gel food coloring. One purple, one black and one green. Gently fold in the color until it is completely mixed through. You will have one bowl of each batter: white, purple, black, green.
Add the colored cake batter to the cake pans. Starting with the white batter (I always start and finish with the white batter so you will have some white on the cake bottom and on the top). Continue with purple, black, green and top with the remaining white batter. Give the pan a little shake (side to side) to distribute the batter around a bit between each color. If you like a swirl effect, create swirls with a toothpick or skewer. However, don't overdo it - since you are using black color gel, you risk getting muddy colors instead of vibrant ones.
Bake 35 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove cakes from the oven and let the cakes cool in the pans for about 15 minutes. Turn onto cooling racks, peel off the parchment paper and allow them to cool completely before frosting.