The night before: Separate the egg whites and place in a covered container. Allow them to sit at room temperature overnight. Zest 1 lemon and spread the zest on a paper plate or paper towel. Allow it to dry overnight.
Line two cookie sheets with parchment paper (circles traced for easy piping).
Sift together the almond flour and confectioners' sugar. Stir in the lemon zest and set aside.
In a large bowl of a stand mixer combine the egg whites and granulated sugar. Turn the mixer to medium (speed 4 on a Kitchen Aid) and whip for 3 minutes. Increase the speed to medium-high (speed 7) and beat another 3 minutes. Turn the mixer up to 8 and beat another 3 minutes.
Add the food color (if using) and whip for one more minute on high speed. You should now have a very stiff, dry meringue. The whisk attachment will hold a large clump of meringue in the center. If not continue beating for a minute at a time until it does. Remove the meringue from the whisk attachment and set the whisk aside. Add the dry ingredients all at once and fold them in with a rubber spatula. Gently fold until all the dry ingredients are incorporated and you have a shiny smooth batter. If you spoon some out and drop it back into the mix, it should blend back into the remaining batter within about 45-60 seconds. If it remains as a dollop on top, fold the batter a few more times.
Spoon some of the batter into a piping bag fit with a ½ inch round tip. Pipe the batter into the pre-traced circles on the baking sheet. Stop piping before the circle is filled as it will spread a little on its own.
Rap the baking sheet firmly on the counter or floor to remove any air bubbles. Allow the macarons to sit at room temperature for 60 minutes.
Bake in a preheated 300°F oven for 18-20 minutes or until you can peel the parchment paper away cleanly. If you pick up a macaron and the top comes off, it's not done. Remove the parchment paper to a cooling rack and cool completely before filling.