While the cake is cooling, prep the icing. If you are making colorful icing, get three to five small bowls and evenly distribute the icing in the bowls. Add 4 to 5 drops of food coloring to each bowl and mix.
Take a piece of plastic wrap (at least 8 to 10 inches long) and put your icings next to each other in parallel lines. Each line of icing should be about 6 inches in length and come to about a half inch from the end of the wrap.
Roll the plastic wrap up so all the colors are next to each other lengthwise, leaving one end open to allow for the icing to come out during piping. Twist the opposite end to keep the icing from coming out the other side.
Fit your piping bag with your piping tip. Then, put your icing wrap into the bag open end first.
Once your cake is completely cooled, pipe your icing along the outside of the cake.
Top with sprinkles, if desired.
