Beat the margarine, sugar and vanilla using an electric whisk until pale and smooth, then mix in the flour until fully incorporated and you have a smooth dough. Bring together with your hands, then divide into three equal pieces (about 200g each). Wrap one piece and set aside, then colour the second piece red by mixing in a drop of food colouring gel, then make the third piece green. Separately wrap, then chill all the doughs for 30 mins.
Roll out the plain dough between two sheets of baking parchment into an approximately 30 x 20cm rectangle, then repeat with the red and green doughs.
Carefully lift the plain dough rectangle over the red, then the green dough over the plain, peeling away the parchment as necessary to create a triple stack of dough layers. Put a clean sheet of baking parchment on top, then lightly roll to help the layers stick together.
Peel away the top sheet of parchment, then use the sheet underneath to help roll the dough from the longest side to make a log. Wrap tightly and chill overnight in the fridge, or for 1 hr in the freezer.
Heat the oven to 160C/140C fan/gas 3 and line two baking trays with baking parchment. Slice the chilled dough into 2cm rounds. Place on the prepared baking trays, with plenty of space between each, and bake for 12-14 mins. Leave to cool completely on wire racks before serving. Will keep in an airtight container for up to three days.