Preheat oven to 350°F. Prepare six 8-inch round cake pans: grease and flour them (or spray with nonstick baking spray with flour) and line the bottom with parchment paper. *See note for how to do this in 2 batches if you don’t have 6 pans*
Add cake mix, flour, egg whites, milk, sour cream, and oil to a large bowl. Mix on medium speed with a hand mixer until smooth.
Divide cake batter evenly into 6 bowls. (If you have a kitchen scale, you can make sure they are even by weighing the batter in each bowl as you divide it.)
Color each bowl a different color (one color per bowl). Stir to distribute color throughout, being sure to scrape the bottom of the bowl.
Pour batter in each pan (one per color).
Bake cake 15-20 minutes, or until a toothpick comes out clean. Cool completely before removing from pans.
Remove the cakes from the pans and remove parchment paper. If needed, level the cakes with a serrated knife.
Place purple cake layer on cake plate. Top with frosting, then add the blue layer, then green, then yellow, orange and red, making sure to frost in between each.
Frost the outside of the cake with a thin crumb coat of frosting. Chill cake until frosting is set to the touch then do a second layer of frosting. Decorate with sprinkles as desired.
Store cake covered in refrigerator for up to 3 days.
Make ahead: you can make the cake layers and frosting and freeze them separately for up to one month. Be sure to wrap layers well in plastic wrap and freeze frosting in an airtight container.