Mix white cake batter according to the box directions, then divide it among 6 bowls, starting with 1/2 cup in the first bowl and working up to 1 1/2 cups in the last. Whisk a different color of gel food dye into each bowl--red in the smallest, then orange, yellow, green, blue, and end with violet in the largest. (Rinse the whisk to prevent color mixing.) Starting with the largest, pour each bowl of batter into the middle of a greased 9-inch round cake pan, spinning it after each addition to distribute evenly. Tent with foil to prevent browning and bake at 325 degrees until cooked through, about 1 hour.