This type of icing is very forgiving, so experiment with consistency. You don't want it too runny or too stiff. If it's too runny, add more sugar, too thick, add more milk.
Remember when you add color that it is easier to put it in than to take it out!
This icing takes a long time to dry after you put it on a cookie, but until then- it turns to concrete in seconds. Keep your containers covered until you are ready to use it.