Pipe lines on cake. Prepare seven 12 in. decorating bags with couplers and tip 10. Fill separately with orange, green, blue, purple, pink, teal and white icing. Reserve bowls for later step.
Using a ruler and toothpick, mark a line around the cake 3¼ in. from the bottom. Do the same 4½ in. from the bottom of the cake. This marks where the colored stripes will appear on your cake.
Helpful Hint: Use the ruler to mark ½ way between the two marked lines (approx. 3 7/8 in.). This is where the center of your rainbow bar will be.
Starting at the bottom of the cake, use bag with white icing to pipe continuous lines from the bottom of the cake up to the first marked line.
Switch to purple icing. Pipe one line around entire cake. Repeat with blue, teal, green, orange and pink icing.
Reserve tinted icing in bags.
Switch back to bag with white icing. Pipe continuous lines to the top of the cake.
Ice and smooth top of cake. Using icing smoother, carefully smooth sides of cake, wiping icing from smoother after each use.
Chill cake until icing is set, about 2 hours.