In a large bowl, combine 4 cups (1 lb.) powdered sugar, 6 tablespoons water, and 3 tablespoons meringue powder (or omit water and meringue powder and use 6 tablespoons pasteurized egg whites). With an electric mixer on low speed, beat until evenly moistened, then beat on high speed until stiff, glossy peaks form. Controlling consistency of icing is simple: If icing is too thick, add a few drops of water; if it's too thin, gradually add more powdered sugar.
Tint icing with food coloring as desired; to make all three cookie designs with Favorite Sugar Cookie Dough, divide icing into four portions and tint three blue, green, or red, leaving one white.