Bake your cupcakes and allow them to cool completely. While the cupcakes are cooling make your frosting.
Divide your buttercream into four equal parts into four bowls. Using your food coloring, color one part blue, one part purple, one part pink and one part yellow.
Place a large piping tip into your pastry bag. Working in small batches scoop the blue buttercream into one side of the pastry bag keeping room for the other colors.
Place some of the purple buttercream next to the blue buttercream.
Place some of the pink buttercream next to the purple buttercream.
Finally place some of the yellow buttercream next to the pink buttercream.
You should end up with each color of the buttercream next to each other in the pastry bag.
Twist the open end of the pastry bag to push the frosting to the tip to make sure that the colors have intertwined.
Begin frosting the cupcake on the outside edge then work to the center of the cupcake.
Refill the bag as need in the same way you filled it the first time.