Prepare your favorite buttercream recipe.
Keep half the buttercream aside and leave white.
Place a small amount of buttercream onto cake board or cake stand to help adhere the first cake layer.
Stack the cake by alternating layers of cake and buttercream.
Frost the exterior of the cake as smooth as possible.
Portion remaining buttercream into 3-6 bowls (depending how many colours you want).
Add 1-2 tsp of food coloring to each bowl and mix (pro tip: use a professional grade gel colour like Chefmaster or Americolor to achieve vibrant results! For very vibrant colors, let stand 1-2 hours between mixing and using the frosting as the color will intensify over time.)
Lay out a sheet of cling wrap on your counter.
Spoon each color into a straight line leaving several inches of cling wrap on each end. Abut each color directly against the last, until you’ve added all your colors.
Roll the cling film into a firm log and twist each end well to close.
Prepare a piping bag with your desired piping tip (pro tip: 1M is the most common tip for this application).
Cut one end of the log and carefully place into piping bag, cut end towards the tip.