Make and tint icing. Prepare buttercream icing following recipe directions. Reserve 3/4 cup icing. With remaining icing, fill, stack and ice cake smooth. Use tip of spatula to ice swirl on top of cake, starting at center and moving outward towards edge of cake.
Divide reserved 3/4 cup icing equally among three bowls. Tint icing green, blue (combining Teal and Sky Blue) and purple (combining Violet, Rose, and Ivory).