Directions for the frosting:
Beat together the sugar and butter until smooth. Add the vanilla and milk and beat to combine. Add more milk, 1 T. at a time, if needed to reach desired spreading consistency. Add desired food coloring and mix thoroughly (you can separate into smaller bowls to make smaller batches of various colors. I usually let my daughter choose the colors for her batch).
Note: I like to make the frosting as the cookies cool, but you can also make it ahead, refrigerate, and then bring to room temperature before spreading.