Grease 2 baking sheets. Beat together the butter and sugar until very pale and creamy. Beat in the flour and vanilla extract until smooth.
Divide the cookie mixture in half and stir the yellow food colouring into one half and the orange colouring into the other. Put the cookie mixture into 2 large piping bags fitted with 1 cm (½ in) plain nozzles. Pipe fingers onto the baking sheets, each about 12 cm (5 inches) long, spacing them slightly apart.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes, or until slightly risen and just beginning to darken. Leave to cool for 5 minutes, then transfer to a wire rack to cool completely.
Beat the royal icing sugar with enough water to give a consistency that just holds its shape. Spread a little icing over one end of each cookie and scatter with plenty of sugar sprinkles and silver balls. Leave in a cool place to set for about 1 hour.