Decorate cake. Prepare buttercream icing following recipe directions. Tint three shades of green: light, medium and dark. Use spatula and medium green icing to ice cake smooth.
Use tip 366 and dark green icing to pipe shell-motion leaves, beginning at bottom border and then around cake. Move tip up 3/8 in. and use tip 366 and light green icing to pipe a row of shell-motion leaves, followed by a row of medium green icing shell-motion leaves. Repeat, alternating icing colors to pipe rows of shell-motion leaves and cover cake sides.