Prepare one baking sheet by placing two macaron templates on the sheet. You can print the templates here. Place parchment paper or silicone mats over the templates. Set baking sheet aside. Place oven rack in the lower half of the oven, but not all the way to the bottom. Preheat oven to 300 degrees.
Add powdered sugar, pistachios, and almond flour to the bowl of a food processor. Process the ingredients for about 30 seconds so that the mixture is super fine. Sieve the flour and sugar mixture three times and discard leftover bits of pistachios. Set mixture aside.
In the bowl of a stand mixer with a whisk attachment, add egg whites and cream of tartar and begin to beat on medium-high until it begins to foam. Once it starts to get foamy, begin to slowly sprinkle the granulated sugar into the egg whites until it's all been incorporated. Increase the speed to high to whip the meringue into stiff peaks. The meringue should have the consistency of shaving cream, NOT soft serve. Add food coloring and mix to combine. If you want a deeper color, add a little bit more coloring. But a little goes a long way.
Once the meringue is done, add one-third of the sifted flour mixture to the meringue and fold with a spatula. Do this step very carefully and slowly, cutting the mixture down the middle and swooping around. Once it's incorporated, add a second-third and gently fold in. Then, add the final third. Mix just until it's incorporated. Do not over mix. The consistency should be like slow-moving lava. To check for consistency, the batter should flow from your spatula like thick lava and hold a ribbon for a couple seconds. If it falls into clumps and doesn't fall slowly, it is not mixed enough. Keep mixing slowly so that the batter deflates a bit and the proper consistency is reached.
Transfer batter to a piping bag fitted with a smooth, round 1/2 inch tip. Holding the bag in two hands directly over the parchment, pipe a small amount of batter into the first circle stencil. Stop pipping right as the batter reaches the edge of the circle. The batter will spread so don't add too much. Move on to the other circles until all the batter is gone. When you get to the end of the batter, a lot of it will stick to the bag. To push it down, use the edge of a bench scrapper.
After you've pipped all the circles, smack the baking sheets on the counter a couple times to release air bubbles. Use a toothpick to pop all the air bubbles. If you don't do this step, the air bubbles will release steam while baking and crack the shells.
Let them sit to dry out until the tops are smooth to the touch, about 30 minutes. They are done when you rub your finger over the top and your finger doesn't stick.
Bake macarons in preheated oven for 14-16 minutes. Make sure to rotate the sheet halfway through, for even baking. Remove from oven and let them cool for 5 minutes before transferring parchment sheet to a wire rack to finish cooling.
While the macarons shells cool, make the pistachio buttercream. (see recipe below)
Lay half of the macarons flat side up on a baking sheet. Cut a little corner off the bag with the buttercream and pipe a small circle of buttercream in the center of one shell. Top with the other shell and gently press just enough to push the filling out to the edges. Do this for all the macaron shells until done.
Once filled, macarons can be refrigerated in an airtight container for up to five days. Or you can freeze for up to five months.