I pulled a little bit of info from 4 pins plus some knowledge I already had to make it work.
Step 1: To make the cake batter use 2 boxes white cake mix + 1 large instant vanilla pudding. Mix these together and then followed the directions on the box. This will yield 16 servings, or 32 1/2 slice servings (which would still be plenty!)
Step 2: Divided the batter into 6 bowls using approx. 1 1/3 cups batter per bowl.
Step 3: Start mixing your colors using gel food coloring. I used the Betty Crocker regular and neon colors. To get the colors vibrant you will need to use a lot of food coloring, about 1/2 a tube per color. I used red, neon orange, yellow, a mix of the green and neon green, blue, and neon purple with a touch of neon pink (purple food coloring on its own always just looks dirty to me, the pink pops the color up).
Step 3: Bake your layers, take a little bit of time off of the recommended cooking time because you made 3 layers per box instead of 2.
Step 5: Start Icing. I used 5 5! tubs of white frosting for this cake and put very thin layers of icing. Make sure to stir each tub of icing before you start suing it, this lightens the frosting up and makes it easier to spread. This cake is deceptively huge! Layer your cakes in rainbow order, just remember ROY G. BIV (without the v!) with a thin layer of icing between each layer.
Step 6: Put a thin (crumb coat) of icing around the whole cake and then pop it into the fridge for a few hours. This lets the first layer of icing set, keeps the cakes cold and firm to make icing easier so they don’t start sliding all around and keeps crumbs out of your final layer of frosting.
Step 7: Put your final coat of icing over the whole cake and pop it back in the fridge. You can be done at this point, or add decorative icing finishes as I did after this layer sets up and hardens.
Step 8 (optional): Add decorative icing. To get the rainbow stars and piping effect stripe your pastry bag with the gel food coloring before you add in the frosting. Don’t go crazy with the decorative icing here or the cake will look a mess, you are just going for an accent, the real star of this cake is the layers inside. Put the cake back in the fridge when done.
Serving the cake: Do not bring this cake out of the fridge until you are ready to light the candles and cut and serve, and plan on cutting quickly! The warmer the cake is when you slice it the harder it will be.
Use a sharp knife ( I used a large chef knife and a bread knife successfully) Between each slice wipe the knife off well or your colors will mix on the edges of the cake and it will look bad. Also any extra frosting cake on the knife will make it harder to cut cleanly. I sliced this cake into 16 pieces, and each piece was huge!
How to cut: cut across the whole cake making 2 halves. then cut 1 of those halves in half. then that 1/4 in half and then each of those slices in half. Then start removing pieces. slicing this cake 1 slice at a time like a normal cake will just give you a big headache and a big mess!
Rainbow cake was inspired by these 4 pins: