Preheat oven to 180°C/160°C fan forced. Line 2 x baking trays with baking paper.
Cream butter and sugar in the bowl of an electric mixer fitted with a paddle attachment until creamy. Beat in egg, juice and rind and food colouring if using. At the lowest setting beat in flour until combined.
Take heaped teaspoons of mixture and roll into balls, then roll in icing sugar and place on trays 3cm apart, repeat with remaining mixture.
Bake trays of biscuits for 15-18 minutes or until crisp but not coloured. Dust with more icing sugar straight out of the oven. Allow to cool on the tray for 5 minutes before transferring to a wire rack to cool completely.