In a food processor, puree almonds with ½ cup icing sugar until very fine and pour into a bowl. Stir in remaining 1 cup icing sugar.
Whip egg whites with salt in a clean bowl until foamy, then add sugar and continue to whip until whites hold a stiff peak. Stir almonds into whipped whites in 2 additions. Add food colouring in small amounts (and split macaroon batter into smaller bowls if you wish to add a variety of colours). Spoon batter into a piping bag fitted with a ½-inch plain tip, and pipe batter into ¾-inch “kisses” onto a parchment-lined baking tray, leaving an inch between each macaroon. Top pink macaroons with a little sprinkle of crushed pink peppercorns and green macaroons with chopped pistachios. Let macaroons sit to dry for 20 minutes.
In the meantime, preheat oven to 300 °F. Bake macaroons for about 20 minutes, rotating pan halfway through cooking, until edges and tops of macaroons are visibly dry but not browned. Let macaroons cool on tray before removing.