This colorful cake is dazzling, but simple. Just follow the directions for placing the colors and the colors will mix themselves. Prepare the cake mix according to directions, or use your favorite white cake recipe. Set aside three 1/2 cup portions of batter and pour the remainder into a 9' by 11' pan that had been greased and floured. To each of the 1/2 cup portions, add 3-5 drops of one food color so that you have 1/2 cup each red, blue and yellow. Next, spoon 1/4 cup of each of the colored batters in the cake mix in the baking pan, making a half circle. Then, repeat the procedure with the remaining 1/4 cup portions (you should have a full circle of red, blue, yellow, red, blue, yellow.) Using a knife, blend the colors together with swirling strokes, so that the colors will touch and mix into colors of the rainbow. Bake the cake as directed. Frost with your favorite frosting or dust with confectioner's sugar.