Step 1: In the bowl of a stand mixer fitted with the whisk attachment, combine the sifted powdered sugar and meringue powder. Mix on low speed until just combined.
Step 2: Gradually add 6 tablespoons of warm water and the clear vanilla extract (if using). Mix on low speed until combined, then increase to medium-high.
Step 3: Beat on medium-high speed for 5-7 minutes, or until the icing is glossy, bright white, and forms stiff peaks when the whisk is lifted. The icing should be very thick at this stage.
Step 4: If you need to thin the icing for outlining or flooding, transfer a portion to a separate bowl. Add additional warm water, 1/4 teaspoon at a time, mixing well after each addition. For outlining, aim for a consistency where a ribbon of icing dropped into itself disappears in 15-20 seconds. For flooding, aim for a '10-second icing' where the ribbon disappears in 5-10 seconds.
Step 5: If coloring, divide the icing into separate bowls and add gel food coloring, mixing until evenly distributed. Gel colors are recommended as they won't thin the icing as much as liquid colors.
Step 6: Transfer the prepared royal icing to piping bags immediately for decorating. Ensure any unused icing is stored in an airtight container with plastic wrap pressed directly onto the surface to prevent crusting.