In a large mixing bowl, start by sifting the powdered sugar to ensure there are no lumps. If you're using meringue powder instead of egg whites, mix the meringue powder with the water in a separate bowl until dissolved before adding it to the powdered sugar.
Using a hand mixer or a stand mixer fitted with a whisk attachment, begin to mix the powdered sugar and egg whites (or meringue mix) on low speed. This initial blending is crucial to avoiding a powdered sugar cloud!
Once well combined, gradually increase the mixer speed to medium-high. Continue beating for approximately 5-7 minutes until the icing reaches stiff peaks. To test for the right consistency, dip a spatula into the icing; it should hold its shape and stand upright without drooping.
Now, gently fold in the lemon juice and vanilla extract (if using) and mix for an additional minute, ensuring these flavors are fully incorporated into the icing.
If you're adding color, pour the icing into small bowls and add food coloring. Mix in the color until you achieve your desired shade; remember, you can always add more color if needed!
Adjust the icing consistency if necessary. If it's too thick, add a few drops of water to thin it down. Conversely, if it's too thin, gradually mix in more powdered sugar until it reaches the optimal consistency for either flooding or outlining your cookies.
Decorate your sugar cookies with the royal icing, taking care to allow sufficient drying time between flooding and outlining if utilizing both techniques for more intricate designs.
Notes
Consider letting the decorated cookies sit for a few hours to allow the icing to set, creating a beautifully glossy finish. Arrange them on a decorative platter for a stunning display!