Pour water in the bowl of a standing mixer or a large mixer (if using a hand mixer). Gently swirl the water around to coat the sides of the bowl (this will help to prevent the dried ingredients from sticking).
Add meringue powder and powdered sugar all at once.
Start mixing on low, scraping down the bowl as needed, until everything is evenly combined.
Increase speed to medium-high and beat for 1 minute.
Continue mixing on medium-high in 30 second intervals, stopping to check the consistency after each interval. To check the consistency, use a spatula or spoon to scoop and drizzle some icing over the surface. Watch the design you drew as you count to 10. The icing is done when it takes 10 seconds for the surface of the icing to turn completely smooth.
Divide icing between bowls and stir in icing colors as desired. Transfer to piping bags. Icing can be refrigerated for up to 2 days or frozen for up to 2 months.
Make sure that cookies have cooled completely before using the frosting to decorate. Let decorated cookies sit at room temperature until the icing is fully set (this can take 1 to 4 hours, depending on the temperature and humidity of the room).