This versatile icing is an essential part of any decorator's repertoire. When thick and dense, it is good for piping; thinned with water, it is used for flooding. Always give it a good stir before putting the icing in a pastry bag.
In the bowl of an electric mixer fitted with the paddle attachment, combine all ingredients on low speed. Mix until icing becomes fluffy yet dense, 7 to 8 minutes. Use immediately, or transfer to an airtight container, as royal icing hardens quickly when exposed to air. Stir well with a spatula before using. If tinting, add coloring just before using.