Royal Icing can be tinted before piping when decorating cookies. Don't mix the recipe too much in advance because it will crust or dry out. (Crusting is a thin layer of icing that hardens on top. It can be difficult to remove. If you stir even a small amount into the icing, you ruin it.) This is used with the Cookie Decorating Tutorial 2.
Cookie Decorating Tutorial 2

How to:
1. Prepare Kelly's Royal Icing Recipe.

2. Thin the icing. The consistency is right when the icing flows smoothly off the spoon, in an unbroken stream. It pipes better if it is not so thick.
 
3. Then separate the thinned icing into small bowls before coloring. You may need larger or smaller amounts depending on the color being used. 
Add the desired color and stir till uniform.

4. Cover immediately with a damp paper towel as they can dry quickly. Leave some white in case you need to correct a color.