Decorate cake. Prepare buttercream icing following recipe directions. Tint icing light and dark violet. Position cake on turntable for decorating. Use spatula and light violet icing to ice cake smooth.
Use tip 104 and dark violet icing to pipe ruffle, beginning at bottom border and then around cake. Move tip up 3/8 in. Use tip 104 and light violet icing to pipe a second ruffle, moving tip up 3/8 in. Repeat, alternating icing colors to pipe rows of ruffles, and cover cake sides. On cake top, begin at the outside edge and pipe ruffle around top. Move tip in 3/8 in., repeat piping alternating colors to cover cake top.