Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until it is smooth and creamy– about 1 minute.
Add the vanilla and mix to combine.
With the mixer on low, gradually add the confectioner's sugar. Scrape down the sides of the bowl as necessary to make sure all of the sugar gets incorporated.
Turn the mixer to medium high speed and beat until the buttercream becomes light and fluffy– about 2 minutes.
Divide the frosting in thirds and place one-third of the icing in a small bowl - set aside. This will be the white.
Place another third of the frosting in a small bowl and add several drops of red food coloring. Mix until the color is disbursed throughout and you have achieved the desired shade of red. Set aside.
Add several drops of green food coloring to the last third of frosting and mix to combine.
Transfer all three colors of the icing to a piping bag fitted with a star tip - making sure to use a thin spatula and fill the frosting in stripes or sections inside the bag to make the 3 colors.
Swirl the frosting on top of the cupcakes - this should come out with all three colors swirled together.
Add shimmer dust and/or sprinkles if desired.
Enjoy the cupcakes immediately or store them in an airtight container in the fridge for up to two days.