Once you’ve piped an outline on a cookie, the next step is to “flood” it with color. The dried lines of royal icing act as a dam to catch the wetter icing as it’s piped over the surface. Add a teaspoon or two of water to the royal icing you used to pipe the design, and put this wetter icing in a piping bag with a narrow tip. You can also put the icing into a zipper sandwich bag and cut off a corner with scissors to let a thin stream flow.
Flood the cookie with icing, using a toothpick as needed to nudge the icing into any edges or corners. Keep the flood icing within the lines—pipe it on in a smooth stream but give it time to spread across the cookie before adding more so you don’t overdo it! Allow each cookie to dry thoroughly before storing.