Colored decorating icing: yellow, orange, red, purple, green blue (recommended: Cake Mate).
Preheat oven to 350 degrees F. Lightly spray 2 (8-inch) round cake pans with cooking spray; set aside.
In a large mixing bowl, beat together cake mix, water, oil, egg whites, and vanilla on low speed using an electric mixer for 30 seconds. Scrape down sides of bowl using spatula and beat on medium speed for 2 minutes.
Divide batter into the prepared pans evenly and bake in the oven for 32 to 36 minutes or until a toothpick inserted into the center of the cake comes out clean. Use oven mitts to remove pans from oven and let cool for 10 to 15 minutes before turning over onto wire racks to cool completely. Trim off the tops of the cakes, to make a smooth surface.
Place 1 tablespoon of each flavor of gelatin in separate small bowls. Stir 1/3 cup boiling water into each and stir until dissolved. On the trimmed sides of the cakes, splash the different gelatin all over the cakes. Cover cakes, separately, with plastic wrap and refrigerate for 3 to 4 hours.
Remove cakes from refrigerator. Unwrap the cakes and place 1 cake on a plate or stand and spread white icing evenly on top. Place the second cake on top and finish icing cake, top and sides with the white icing. Randomly place big dollops of the colored decorating icing all over the iced cake. Using a small spatula, swirl the dollops together to create a tie-dye/rainbow effect on the cake.