Mix the sugar with 1 tablespoon of the lemon or orange juice in a bowl. Gradually add the remaining juice, stirring well with a wooden spoon, until the icing holds its shape but is not difficult to spread – you may not need all the juice.
Reserve 3 tablespoons of the icing and spread the remainder over the tops of the cooled cakes using a small palette knife. Stir a little extra sugar into the reserved icing to thicken it until it just forms peaks when lifted with a knife. Put in a piping bag fitted with a writing nozzle, or use a paper piping bag with the merest tip snipped off (see
Colour half the buttercream with pink food colouring and the other half with lilac food colouring. Put in separate piping bags fitted with star nozzles.
Pipe rows of pink and lilac buttercream and white icing across the cakes.