Decorate cake. Spatula ice cake smooth using white icing. Using 9 in. angled spatula, apply ribbon of mint green icing around bottom of cake, about 1 in. high. Using 13 in. spatula, swipe cake smooth, slightly blending mint green and white icing.
Prepare two decorating bags separately with tips 1G and 2C. Fill both bags with remaining mint green icing.
Pipe large 1G rosette on top edge of cake. Pipe two 2C rosettes on either side of large rosette, reducing size and pressure.