Fill in lines. Prepare seven decorating bags separately with pink, orange, yellow, green, teal, blue and purple icing. Cut ½ in. off tip of each bag.
Alternating colors, fill in notches on sides of cake. Using icing smoother, start at bottom of cake and swipe up, smoothing each side. Repeat, cleaning the smoother off before each swipe, until sides are smooth.
Using icing smoother or spatula, carefully swipe excess off of top of cake.
Reserve remaining icing.