Make your favorite vanilla cake recipe according to the cake instructions. Divide the batter into four bowls and set aside.
Place the baking pans onto parchment paper and, using a pencil, trace the outside of the bottom of each pan. I usually wipe the inside bottom of each pan with a little pat of butter, just to keep the paper in place, then place one parchment paper circles onto the bottom of each pan.
Sprinkle each with a light dusting of powdered sugar. This helps ensure that the cake doesn't stick to the pan or the parchment paper. I have found that when I just use parchment paper on its own, the cakes tend to still stick.
Once you've chosen the icing colors you would like to use, place a small toothpick full of color in one of the bowls containing batter. Then in the next bowl, place double that amount of color. And so on, until you get a noticeable difference in color between the bowls of batter. Completely swirl the gel coloring into the batter as evenly as possible.
Next, simply pour each bowl into a separate prepared baking pan. Bake according to your cake recipe's directions. While the cakes are baking, make your favorite frosting recipe.
After each layer is done baking, allow the cakes to cool completely at room temperature. Once the cakes are cool to the touch, remove the parchment paper. Use a cake turntable to make decorating easier. Start by stacking the cake layers from lightest to darkest color, or vice versa. It's up to you what order you stack the colors!
As you stack, frost the top of each layer with a thin layer of frosting.
Once the cake is assembled, frost the outside of the cake and decorate as desired.
Notes
Cake: Two boxes of your favorite cake mix should make 4 cake layers (2 layers per box). Frosting: One or two cans of your favorite storebought frosting should work. Or feel free to use my favorite Cream Cheese Frosting recipe!