1. Preheat your oven to the temperature specified by the cake mix or your favorite white cake recipe's directions.
2. Mix the cake as your recipe tells you to. If you can't find a recipe for a white cake you may use a yellow cake recipe/box, all you need to do is add only the egg whites.
3. Separate the cake batter into six small bowls. You will want to add slightly more batter to two bowls compared to the other four.
4. Decide what colors you want to use. If you are doing a rainbow cake you will need red/pink, orange, yellow, green, blue, and purple. Add the gel food coloring to the correct bowls, mixing as necessary to create the colors. Make sure to do your red/pink and your purple in the two bowls that have more batter than the others. Since these two are the end colors you will need more of them.
5. Grease your 8 or 9" round cake pans.
6. Pour half of the red/pink mixture into pan 1. Don't worry about getting it spread evenly across, just pour it into the center of the pan.
7. Pour half of the orange mixture into pan 1 directly in the center of the red/pink. The weight of the batter will push the previous layer out into a wider circle.
8. Continue until you have added half of all of your mixtures into pan 1. Let it settle on the counter.
9. Repeat with pan 2, only this time you will reverse the color combination. Add the rest of your purple mixture, then blue, etc... until you have added all 6 colors into both cake pans.
10. Let the two cake pans settle and the batter spread. You can shake them side to side gently to help them along. Don't worry if your pans aren't perfect, concentric circles.
11. Bake them as directed on the box or reicipe. Keep a careful eye on them as they may start to brown. If this happens lower the heat on your oven a bit and tent a piece of aluminum foil over the cake pan.
12. Use a toothpick or a knifeto test that they are ready. If it comes away clean then you can remove them from the oven.
13. Let the cakes sit undisturbed for several hours (preferrably overnight) until they are completely cooled. If you move them too much while they are still hot then the cake may crumble or split.
13. Once the cakes are cooled stack them, ice, decorate, and watch your friend's/family's faces when you cut into their cake!