Divide the marzipan into three equal pieces. Add different amounts of food colouring to each piece, kneading until evenly coloured. The idea is to get three different shades of green.
Thinly roll out each piece of marzipan on a work surface dusted with icing sugar. Cut into equal-sized squares.
Mix a couple of tablespoons of icing sugar with a little water in a small bowl to make 'glue.' You want the mixture to be like thin cream.
Dip the back of the marzipan squares in the icing sugar mixture and glue to the cake. Working from the outside to the centre, randomly alternate the colours of green. In the middle, leave two squares blank for the Creeper's eyes. Cut pieces of marzipan smaller to form the details of the mouth.