Grease and line 2 baking sheets with nonstick baking paper.
Put the icing sugar in a food processor with the ground almonds and blend to a very fine consistency.
Put the egg whites in a thoroughly clean bowl and whisk until stiffly peaking. Gradually whisk in the caster sugar, a tablespoonful at a time and whisking well after each addition, until thick and very glossy. Divide the mixture equally between 2 bowls and add a few drops of food colouring to each bowl. Divide the almond mixture equally between the 2 bowls and use a metal spoon to stir the mixtures gently to combine.
Place 1 colour in a piping bag fitted with a 1 cm (½ inch) plain nozzle and pipe 12 x 3 cm (1¼ inch) rounds onto 1 baking sheet. Tap the baking sheet firmly to smooth the surfaces of the macaroons. Wash and dry the bag and piping nozzle and pipe 12 rounds in the second colour onto the other baking sheet. Leave to stand for 30 minutes.
Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for about 15 minutes, or until the surfaces feel crisp. Leave to cool before carefully peeling away the paper.