Add room temperature egg whites and cream of tartar to a stand mixer bowl fitted with a whisk attachment. Whip on medium-low speed until the eggs are very foamy, about 3 minutes. Add a third of the granulated sugar at a time, allowing the mixer to beat in each batch of sugar for about 45 seconds before adding in another batch.
Once all the granulated sugar is incorporated, turn the mixer speed to medium-high and allow the egg whites to continue to whip until stiff peaks form, about 10 - 11 minutes. You'll notice the meringue starts to ball up inside the whisk and is glossy. To test the meringue stiffness, pull the whisk out. A stiff peak should stand straight up (no curl at the tip). You'll also know it's ready because you can fully flip the bowl over and nothing moves or falls out. Make sure not to overbeat the meringue at this stage (this is when the egg whites look dull or curdled).
Pour the sifted almond flour & confectioners' sugar into the meringue mixture and gently fold them together using a silicone spatula. Continue until all the meringue is fully incorporated and no dry bits of almond flour remain on the bottom of the bowl.
Add 5 - 10 drops of gel food coloring. Then, begin working the batter until everything is fully incorporated (this is called the macaronage process). Slowly spread the mixture along the side of the bowl, then wipe it back down. Be sure to do this gently. Repeat until the mixture has reached a smooth, shiny, flowing consistency. You can test this by lifting your spatula and seeing if the batter drops almost like lava. Once the batter drops back into the bowl, it should take about 10-15 seconds for the ribbon that formed to "disappear" back into the rest of the batter. Don't over macaronage the batter or the macarons will not properly bake in the oven.
Pour the batter into the prepared piping bag. Hold the bag vertically, and gently pipe 1.5 inch circles. Once you have reached the desired size, release the pressure on the bag and twist your wrist without lifting up. Continue until no batter remains.
Bang the sheet pan down the counter a few times to release any air bubbles. Pop the air bubbles with a toothpick. Allow the batter to sit on the counter and dry for 60+ minutes until a thin "skin" develops, and you can touch the tops without ruining the macaron. This resting time will vary depending on the humidity (they may take up to 90+ minutes to dry if you live in a humid or rainy environment). While the macarons are drying, place the oven thermometer in the center rack of the oven. Heat oven to 300°F (150°C).
Bake one sheet at a time on the center rack of the oven for 19 - 23 minutes. You can test that the macarons are done by gently touching the edge and the feet of the macarons. If the shell wiggles, is sticky, or wet, the macarons need to bake longer. Remove macarons from the oven and move the silicone mat or parchment paper to a cooling rack until completely cooled.
Match up macaron cookies so they are paired with a similar size/shaped cookie. Pipe your desired filling on one side of the macaron shell. Gently place the matching shell on top of the filling.
Mature the macarons with the filling in the refrigerator overnight. This will allow the macarons to slightly soften and develop a deeper flavor.