In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, combine first three ingredients and mix on low speed until glossy and fluffy, 7-8 minutes. To color, place some icing in a small bowl or plastic cup, and stir in a few drops of food coloring until desired shade is reached. Royal Icing hardens quickly when exposed to air, so use immediately, or transfer to an airtight container; it will keep overnight at room temperature. Beat well before using.
*Production note: If you're doing multiple batches of dough, you'll need more icing. An easy way to do this efficiently is to make the icing, one batch at a time, and pour into an airtight container. Make several batches of icing. Then, using a ladle or 1/2 cup measure, place a small bit of the icing in a plastic cup. Mix in food coloring to desired shade, and fill a pastry bag. Repeat until you have all the colors you need. To keep the icing from hardening, place bags tip down in a bowl that has a damp paper towel in the bottom.
**Remember, place the decorated cookies on a tray and leave out overnight, uncovered, to harden. The next morning, package in food-safe cellophane bags or cookie tins.