Remove about 1 cup from the buttercream. Add neon green food gel to the 1 cup, and neon purple food gel to the remaining larger portion of frosting. Mix the colors in thoroughly.
To decorate the cupcakes, using the purple frosting apply a nice layer on each cupcake. Then, using your piping bag and piping tip pipe on the Hearthstone swirl using the geen frosting. If you are not using a piping tip and bag, simply cut the corner off a ziploc baggie and pipe on the design. If you do use a biggie the design won’t be as uniform but it will still look good and taste delicious!