Decorate sides of cake. Using toothpick and ruler, mark diagonal lines on side of cake. Lines should be at 45 degree angle, about 1 in. apart. Repeat on all sides of cake.
Using prepared decorating bags, fill in icing on sides of cake, alternating color pattern to form triangles on corner edges.
Using spatula, work with one color at a time to lightly smooth lines, starting at bottom, then sweeping spatula on diagonal to top of cake.
Using spatula, clean off top edge of cake.