Mix sugar, water, and meringue powder with a mixer on low speed until smooth, about 7 minutes. Icing should be the consistency of glue. (If icing is too thick, mix in a little more water. If icing is too thin, add more confectioners' sugar.) Add food coloring, 1 drop at a time, until desired color is achieved; if making more than 1 color, divide icing and work in batches. Use immediately, or refrigerate for up to 1 week. Stir before using.