Set the egg whites aside to come to room temperature.

Measure the almond meal, powdered sugar, and salt into the bowl of a food processor.  Process the mixture finely, then sift, discarding any large bits.  Set aside.

Measure the granulated sugar in a small bowl.  

Add the cream of tartar to the whites, and mix on medium-high speed, using the whip attachment.  

When the whites hold soft peaks, start adding the granulated sugar, very slowly, while continuing to whip.  

When all the granulated sugar has been incorporated, continue to whip until stiff.  Add color, if using.

Add the almond meal mixture to the meringue, and fold together, until the batter drops from the spatula in a long ribbon.  

Transfer to a piping bag fitted with a large round tip, and pipe 1 1/2-inch diameter rounds onto parchment-lined baking sheets.  

Allow to dry for 45-60 minutes, or until thin, dry membrane forms on the surface.

Preheat the oven to 300 degrees F, and bake the macarons (one sheet at a time) until set (approximately 10-15 minutes).  Do not brown.  

Allow to cool on the baking sheet, then carefully peel from the parchment and sandwich with filling.