Preheat oven to 100°C. Line three oven trays with baking paper. Process icing sugar and pistachios in a food processor. Sift into a large bowl. Process any large pistachio pieces again and return to bowl.
Beat eggwhites in the small bowl of an electric mixer for 1 minute until white but not thick. Gradually add caster sugar and beat for 2 minutes, until the consistency of yoghurt. Add to dry ingredients and gently stir in green food colouring. Do not whip or the mixture will lose volume.
Fit a large piping bag with a 1cm plain tube and fill with mixture. Pipe 3cm rounds onto trays, leaving 3cm between each one. Bake for 15-18 minutes or until smooth and crisp on top but still soft underneath. Cool completely on trays.
To make icing, sift icing sugar into a bowl. Gradually stir in enough milk to make a smooth, stiff icing. Tint with pink food colouring. Sandwich macarons together with icing.