Preheat oven to 350 (or what cake package reads). Using non-stick spray, coat all of your 8″ rounds (you’ll need 6, or re-use what you have).
Mix cake according to directions. Divide batter equally into six different bowls. Using gluten-free food coloring, dye each bowl the following color: purple, blue, yellow, red, orange, green. Pour mixes into 8″ rounds. Bake 2 or 3 rounds at a time, and put the remaining batter into the refrigerator until it’s time to cook.
Allow each cake to cool completely. Prepare frosting while cakes are cooling.
Using a stand mixer or a very strong hand mixer (highly recommend the former), beat butter until smooth. Slowly add powdered sugar and mix thoroughly between each addition. Add salt, vanilla, and milk incrementally until frosting is the desired texture. If you want to add a crumb coating (I did not but this will make it look better) set aside 1 cup of frosting in medium bowl. Continue to add milk and mix until the frosting in the bowl is very thin. This will be your crumb coating frosting.
To stack and ice the cakes, place the purple round on the bottom of your cake stand or platter. Using a serrated knife, level the top of the cake. Either frost the top of the purple layer with frosting, or if you’re doing a crumb coating, add a layer of the thinned frosting and allow to dry before adding 1/4 inch of frosting. Repeat with each layer in this order: blue round, green, yellow, orange, and red.
After each layer has 1/4 inch of frosting, use a spatula to ice the outside and top of the cake. Smooth the edges with spatula or the edge of a knife. Add sprinkles and serve.