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.
Once you're happy with the look, finish decorating the cake with a splatter of rose gold paint & gold sprinkles if desired.
Slice and serve at room temperature.