Preheat the oven to 160°C/fan 140°C/gas 3. Line 2 baking sheets with baking paper.
Place the icing sugar and ground almonds in a processor. Whizz to a very fine mixture. Sift into a bowl, discarding any hard lumps.
Place the egg whites in a clean, grease free bowl. Add a pinch of salt. Whisk to soft peaks, then gradually whisk in the caster sugar until thick and glossy. Add a few drops of orange colouring.
Fold half the almond and icing sugar mixture into the meringue. Mix well. Add the remaining half and fold the mixture, using a metal spoon or spatula, until it is a shiny, thick, ribbon-like consistency. Spoon into a piping bag with a 1cm plain nozzle.