Beat together the butter, icing sugar, milk and vanilla until smooth and pale. Put half the buttercream in one bowl and leave as it is, then divide the other half into 3 bowls. Use the food colouring to dye one pink, one pale green and one pale blue, then transfer them all to one large piping bag fitted with a large star nozzle, including the plain. Pipe swirls on top of the cupcakes – as the buttercream comes through the bag, the colours will start to blend and change as you pipe. Decorate the tops of the cupcakes with your favourite sprinkles if you like.