Heat oven to 350 degrees F. Using sheets of wax paper or parchment paper, trace along the edge of an 8-inch round pan. Do this 6 times and cut out round shapes. Spray the bottom of 3 (8-inch) round cake pans with baking spray. Place one sheet of cut out paper into each of the 3 round pans (or however many pans you have), press down to make sure the paper is flat. Spray the sides with baking spray and set aside.

In a large mixing bowl, add one cake mix, 1 cup water, 1/2 cup vegetable oil and 3 eggs. Mix together until thoroughly incorporated, with a rubber spatula, about 1 or 2 minutes. Measure about batter evenly, you should be able to get about 1 1/4 to 1 1/3 cups batter for each pan.

Using the food colors (in this order: purple, blue, green, yellow, orange, red) and working on one color at a time, drop 1 or 2 drops of gel coloring at a time into one bowl or measuring cup. Using a spatula, mix the batter. Add more color if necessary until you get the shade you want. Tint the divided batter for the first three colors.

Pour the colored batters into separate cake pans. Make sure batter is spread evenly across the bottom of the baking pan by tilting it. Then, tap the cake pan on the counter a couple times.

Bake for 15 to 16 minutes on the middle rack at 350 degrees F. Use a toothpick inserted in the center of the cake to measure doneness, there should be no wet cake mix visible and the toothpick should come out clean or with a little bit of crumbs. Allow cakes to cool for 5 to 10 minutes in the pan. Being very careful because the pans will be pretty warm or hot, flip the cake pans over cooling racks. Remove the paper and discard. Allow cakes to cool completely.

Allow the cake pans to cool a little bit and then wash them. Bake and cool remaining 3 cake layers as directed repeating steps 2 to 5 with the remaining colors.

Place each cake on a flat surface (I put them on parchment paper on my kitchen counter) and carefully trim rounded tops off cakes to level them (but this is optional).

On a serving plate or I used a cardboard cake round that's covered in parchment paper, put a little bit of frosting on the plate. Place the purple cake layer (with the smooth side or bottom side UP) and push down with your hands to ensure the cake stays put.

Add about 1/4/ to 1/3 can of frosting to the top of the cake. Frost the layer with frosting to within 1/4 inch of edge with an offset spatula. Repeat with blue, green, yellow, orange and red cake layers.

Add frosting to the top of the cake and smooth it out. Using your spatula, even out the sides with frosting, add a light coat if needed. This will seal in the crumbs and it's okay if any cake is showing at this point. Add more frosting to the sides and spread evenly with the offset spatula.

Refrigerated the cake for at least 4 hours or overnight before cutting. Add nonpareil decors and serve.