If using the powdered egg whites or meringue powder and warm water, combine them in the bowl of a stand mixer or in a large mixing bowl. Let stand, whisking frequently, until the powder is dissolved, about 5 minutes. If using fresh egg whites, just put them in the bowl.
With the whisk attachment on a stand mixer or with a hand mixer, begin mixing on medium speed until frothy. Add the confectioners’ sugar and continue beating until blended. Increase the speed to high and beat until the mixture is thick and shiny, about 3 minutes for fresh eggs and 5 minutes for powdered. Stir in food coloring (if using). Put a damp paper towel directly on the icing to keep a skin from forming. If not using within 2 hours, cover the bowl with plastic and refrigerate.