Bake your biscuits (we have done a mixture of circle and star shapes) and leave them to fully cool on a cooling rack. In the meantime make up four lots of icing (to a fairly thick consistency) by mixing icing sugar with a drop of water. If your icing becomes too runny add a touch more icing sugar. Colour each lot of icing using a few drops of black, dark purple and pink food colouring and leave one batch of icing white.