Start by splitting the icing into three bowls. The white icing is in one bowl. In the second bowl, mix in the red gel food coloring, and in the third bowl, mix in green gel food coloring.
Spoon some of each of the icings into a piping bag with a small #2 decorating tip. This will be used to outline the cookies. Set them aside.
For the remaining icing, add a small amount of water to thin it out. This will be used to flood the cookie. Put the icing into another piping bag, but this time just cut a small hole at the tip. Set aside.
Use the piping bag with the #2 tip to carefully outline the edges of the cookie with the white icing. After you’ve outlined the cookie, use the second bag of white icing to fill in the inside. Using a toothpick, gently spread the icing to the edges to make it smooth.
Dip the cookie into the white sanding sugar, so that they completely coat the icing. Continue this step for the red and green cookies.
Let the cookies sit for 2 hours so the icing can dry and harden. Enjoy!