Spray 2 (8- or 9-inch) round cake pans with cooking spray. Line bottoms of pans with cooking parchment paper, if desired, to prevent sticking. Spoon 1/3 cup red batter into center of each cake pan. Spoon 1/3 cup blue batter into center of red batter in each pan, followed by 1/3 cup white batter in each. Repeat with remaining batter until used up.