Preheat oven to 350F and grease and flour three 6” cake rounds.

In a large bowl, add the cake mix and all the other ingredients, except for the food coloring. Mix well together.

Divide the batter in half - you only need 1/2 of the batter for the 3 cake layers.

Then divide the cake batter into 3 equal portions, and color each of them.

Pour each into prepared cake tins.

Bake for 20-25 minutes, until a toothpick inserted into the center comes out mostly clean.

Cool until ready to use.

Place egg whites and sugar into the bowl of a stand mixer, whisk until combined. 
Place the bowl over a hot water bath on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch or until it reaches a temperature of 165 degrees F on a candy thermometer. 
Place the bowl on your stand mixer and whisk on medium-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins).
 Switch to a paddle attachment. Slowly add cubed butter and mix until smooth. Add vanilla and continue to whip until smooth.

Spread some buttercream on the cake board.

Place one layer of cake on a cake board and place it in the fridge for 5-10 minutes. This will help the cake stick to the cake board and not slip as you stack and frost.

Remove from the fridge.

Top with approximately 1/3 cup of frosting and spread evenly.

Repeat with remaining layers and apply a thin coat of frosting all over the cake for the crumb coat. Chill for 20mins.
 Frost with a generous layer of frosting and smooth the sides.

Place cake in the fridge for 30 minutes.

Once chilled, drag the striped comb to create ridges for the stripes.

Place cake in the fridge for another 45 minutes to 1 hour until buttercream is completely firm.

While the cake is in the fridge, color the buttercream for the stripes. Color 1/2 cup of frosting for each of the 4 colors - yellow, green, purple, and red. Place each color into separate piping bags.

Once the cake has chilled, remove from fridge and place on the cake turntable.

Pipe the ridges with the colored buttercream, ensuring ridges are filled.

Once all the ridges are filled, using a bench scraper smooth the buttercream, wiping the bench scraper after each swipe. Keep going until stripes are revealed.

Use the leftover buttercream and a 2D piping tip to create the top.