Add the powdered sugar and meringue powder to the bowl of a stand mixer fitted with the paddle attachment (hand mixer will work too). Mix on low for 30 seconds to combine.
With the mixer on low, slowly add the room temperature water then add the almond extract. Increase the mixer to medium-high speed and beat until stiff peaks form (4-5 minutes). When you lift the paddle out of the icing the frosting should create peaks.
To make several colors: separate the icing evenly into the amount of colors needed (if you want 4 colors, divide evenly between 4 bowls). Add the gel food coloring until your reach your desired color hue.
To make the border icing: To one color of icing (let's say green), add water 1 teaspoon at a time until your icing thins a bit. When you pull the spoon out it should still hold its peak but not be too thick to pipe out of a bag. This is your border icing for the outside of the cookies. Remove half of the icing from the bowl and place in a piping bag or icing bottle.
To make the fill icing: To the same green icing, add water 1 teaspoon of water at a time to thin the consistency even more. When you lift a spoon out the icing it should drizzle back down and smooth out in 5-10 seconds. If it takes longer than that, add a bit more water. Place this icing in a piping bag or icing bottle next to the border icing. You will end up with 2 consistencies for each color. If you're making 4 colors, you'll have 8 piping bags/bottles. It helps if you label the bags before adding the icing to them.
Ice your cookies: use the border icing to outline the cookies then use your fill icing to flood the middle of the cookie. The border acts as a dam to keep the flood icing in. If you want details to flood into the cookie you can add them now or place the flooded cookies in the fridge for a few minutes to help them set then add more border colors on top. Get creative, they don't have to be perfect!
Store in an air tight container for up to one week.