Divide the buttercream between 2 bowls. Add the food colouring to one bowl and mix well. Spread the pink buttercream over the tops of the cooled cakes to within 5 mm (¼ inch) of the edges using a small palette knife, doming it up slightly in the centre.
Put half the uncoloured buttercream in a piping bag fitted with a writing nozzle and the remainder in a piping bag fitted with a star nozzle. Pipe lines, 1 cm (½ inch) apart, across the pink buttercream, then across in the other direction to create a diamond pattern.
Pipe little stars around the edges using the icing in the other bag. Decorate the piped lines with silver balls.