Since the tried and true macaron shell recipe is best left alone, most of the flavor in any French macaron will come from the fillings. And the sky's the limit! If you do want to infuse the shells, it's best to stick with a small amount of concentrated, alcohol-based extract, instant espresso, or dried fruit powder. Or you can whisk a tiny bit of citrus zest into the dry ingredients, then sift it out. This allows the citrus oils to transfer to the almond flour. But prevents excess moisture from ruining the meringue.