Place three-quarters of the remaining frosting into a separate bowl and use a generous squirt of gel food coloring to color it a bright shade of yellow. Then scoop the yellow frosting into a large piping bag fitted with a large round tip (like a
Wilton 1A). Seal the top with a rubber band or clip, and set aside.
If you don't have that type of piping tip, you can cut a 1/3-inch opening at the base of the piping bag.