Combine all the ingredients, except the food coloring, in the bowl of a standing mixer fitted with the paddle attachment. Mix slowly until stiff enough to form peaks. The icing should be pure white and thick, but not fluffy and bubbly. If the frosting is overbeaten, it will get aerated which makes it harder to work with. If this happens, let the icing sit to settle, then use a rubber spatula to vigorously beat and smooth it out.
Alternatively, combine ingredients in a large bowl, and beat with hand beaters on low speed until the frosting thickens to stiff peaks.
Add up to 1 tablespoon food coloring and mix with a rubber spatula until the color is uniform. (Adding too much color reduces the sheen of the frosting and can break down the consistency of the frosting over a couple of days.) Store icing, covered, with plastic film on the surface of the icing.
Makes: 3 1/2 cups royal icing