Combine confectioner's sugar and almond meal in the bowl of a food processor and pulse several times until combined and powdery. Sift the mixture into a medium-sized bowl using a tamis or strainer. Use a flexible scraper or rubber spatula to force as much of the mixture as you can through the tamis. You should only have about 1 tsp. of the mixture left in the strainer, which can be discarded.
Make the meringue: In the bowl of a stand mixer fitted with a whisk attachment whip the egg whites with the salt on medium speed until foamy. Reduce the speed to low and stream the granulated sugar into the bowl with the mixer running. When the sugar has been added, turn the speed up to high and whisk the meringue until it holds firm, shiny peaks. Add food coloring at this point, a little at a time and adjust to desired color.
With a flexible spatula, fold the almond mixture into the egg whites until completely incorporated, scraping along the sides and then up and under the middle of the bowl several times. Avoid leaving streaks of meringue or unincorporated almond mixture – this can lead to cracking in the macaron shells. When fully incorporated the mixture should have the consistency of magma – if you form a peak, it should slowly sink back into the batter. Avoid over-mixing the batter or it will become too runny and the shells will not rise properly.
Using a piping bag fitted with a small (3/8-inch, 1 cm) round tip, pipe the macarons in small circles, about 1.25-1.5 inches in diameter. Rap the baking sheets a couple of times on a flat surface, and let the piped batter sit for 15-30 minutes. When the cookies are ready there should be a small "skin" on the cookie (your fingertip should come away clean after touching one).
Bake, in a 325°F oven for 10 to 12 minutes. Use a wooden spoon to keep the oven door slightly ajar (this lets steam escape and prevents cracking), and rotate the baking sheet after 5-6 minutes for even baking.
Remove macarons from oven and transfer parchment to a cooling rack. When cool, slide a metal offset spatula or pairing knife underneath the macaron to remove from parchment.
Pair macarons of similar size, and pipe about 1/2 tsp of the filling onto one of the macarons. Sandwich macarons dust the edges of the macarons with the toasted walnuts. Refrigerate to allow flavors to blend together. Bring back to room temperature before serving.
