Preheat oven to 325°F (or follow the temperature listed on your cake mix box). Lightly coat six 8" round cake pans with non-stick spray and line the bottoms with parchment paper. Set aside.
Prepare a triple batch of cake batter according to the directions on the box. Divide the batter evenly among seven small bowls.
Start by placing 1 cup of batter in each, then gradually add equal amounts to ensure even distribution. Use a toothpick to add gel food coloring to each bowl, mixing until the colors are as vibrant as you’d like. (The seventh bowl is just a backup in case one color doesn’t turn out as planned.)
Pour each colored batter into a prepared cake pan. Bake for about 10 minutes, or until a toothpick inserted into the center comes out clean. Once baked, allow cakes to cool in the pans for 5–10 minutes, then turn them out onto wire racks to cool completely.
Once the layers are fully cooled, start assembling. Place your first layer (typically red) on a cake plate or flat surface. Pipe a ¼-inch layer of frosting on top using a pastry bag to keep it clean and even. Add the next layer and repeat the process, stacking and frosting between each one.
When all the layers are stacked, place the cake in the fridge for about 10 minutes to set. Then apply a thin crumb coat to the outside. At this point, you can either stop and enjoy the colorful layers as-is or chill the cake for another 10–15 minutes before applying a final, smooth coat of frosting.
