Beat the food colouring into the buttercream and then spread all over the tops of the cooled cakes using a small palette knife.
Knead the white icing on a work surface lightly dusted with icing sugar, then roll out. Cut out 12 rounds using a 6 cm (2½ inch) cookie cutter and gently press one on to the top of each cake.
Roll out the red icing and cut out numbers of your choice for 6 of the cakes using a small, sharp knife or alphabet cutters. Secure to the cakes with a dampened paintbrush. Cut out the remaining numbers from the blue icing.
Brush the edges of the white icing with a dampened paintbrush and scatter over the sugar strands.