Beat together sugar, meringue powder, and water in the bowl of a mixer fitted with the whisk attachment on medium speed until smooth and opaque white, about 7 minutes. If icing is too thick, add more water, 1 teaspoon at a time; if too thin, beat icing 2 to 3 minutes more, or add more sugar, 1 tablespoon at a time.
Divide icing into batches, and mix in a different shade of food coloring to each to tint. Use immediately or store in refrigerator overnight. Stir before using.