Beat powdered sugar, meringue powder, and 10 Tbsp. warm water at high speed with a heavy-duty electric stand mixer, using whisk attachment, 5 minutes or until glossy. Stir in up to 2 Tbsp. more warm water, 1 tsp. at a time, until mixture reaches desired consistency. Stir in food coloring, if desired. Use immediately, or store at room temperature in airtight containers up to 1 week.
Elegantly Ice with Ease Using Thin Royal Icing with a wet consistency, pipe a thin border around the edge of the cookie. Fill in the center of the cookie with an even layer of icing. To smooth the surface, swirl a wooden pick in a circular motion around the entire pool of icing, starting at the edges and moving toward the center. Immediately pipe another color of icing, the same consistency as the first, onto the base layer of icing. Manipulate the design gently with a wooden pick.