While cookies are cooling divide the frosting into 4 (or more) small bowls. Put one or two drops of food coloring in each bowl and stir until color is uniform. I use yellow, red, blue and green food coloring. You can make a purple by putting one drop of red and one drop of blue food coloring into frosting.
When cookies are completely cool frost cookies. Top with decorative candies. Allow cookies to "dry" so the frosting doesnt stick to everything.