1st, u beat the egg white and lime juice in a bowl, with a little of the icing sugar. Gradually blend in the remainder to make a paste that just holds its shape. If u want to make different colours (for writing/piping purposes), just turn half the mixture into a separate bowl and colour it as desired.
Meanwhile, thin the remaining white icing with a dash of water, or extra lime juice, until each thickly coats the back of the spoon. Spoon each icing over the tops of the cupcakes (or cakes).