Place cooling rack on large cookie sheet; place doughnuts on rack. To make a rainbow on each doughnut, pipe one stripe of red, orange, yellow, green, blue and purple icing over top and slightly over edge of doughnut so that colors just touch. Pipe 1 or 2 doughnuts at a time so icing can blend together slightly before setting. The icing will spread slightly and drip down sides. Let stand at least 30 minutes or until icing is set. Store in tightly covered container.