1 lb + scant 1/4 cup confectioners' sugar
5 Tbsp meringue powder
1/2 cup cool water (add more, a teaspoon at a time, if needed for desired consistency)
A few drops of paste food coloring
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, combine first three ingredients and mix on low speed until glossy and fluffy, 7-8 minutes. To color, place some icing in a small bowl or plastic cup, and stir in a few drops of food coloring until desired shade is reached. Royal Icing hardens quickly when exposed to air, so use immediately, or transfer to an airtight container; it will keep overnight at room temperature, or in the refrigerator for 2-3 months. Beat well before using.
If you're doing multiple batches of cookies, you'll need more icing. An easy way to do this efficiently is to make the icing, one batch at a time, and pour into a large airtight container. Make several batches of icing. Then, using a ladle or 1/2-cup measure, place a small bit of the icing in a plastic cup. Mix in food coloring to desired shade, and use a pastry bag to fill your squeeze bottles. Repeat until you have all the colors you need.