Stack, fill and crumb coat the cooled cake with the vanilla buttercream. Use this post for guidance, if needed. Chill the crumb coated cake for 30 minutes.
Transfer 1 1/4 cups of the vanilla buttercream to a separate bowl, to color it green. Add a few drops each of Forest Green, Electric Green, Turquoise, Leaf Green and Super Black, adding a little more of each as needed to achieve a deep, emerald green color.Frost the cake all over with a smooth, final coat of the green buttercream. Chill well, before decorating.