Preheat oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4.
Mix cake mix as directed on box using water, oil and eggs. Pour the cake mixture into a well-greased 33cmx23cm rectangular tin.
Bake in the centre of the oven for 25 to 30 minutes or until rounded knife inserted into the centre of the cake comes out clean. Leave in the tin for 10 minutes. Run a knife around sides of the tin to loosen the cake; remove from the tin and transfer to cooling rack. Cool completely, about 1 hour. For easier handling, refrigerate or freeze cake for 30 to 60 minutes or until firm.
Place the icing into a large bowl and stir in the green food colouring until you reach your desired shade of green. Cut the rounded top off cake with a serrated knife to level the surface; place cut side down. From the middle of one short side, make a diagonal cut to each corner to the opposite side, making 3 triangular shapes. Make the tree shape by arranging 2 outer pieces with the straight 33cm sides together on a foil-covered tray. Ice the top with some of the icing. Place remaining centre piece of cake on top of the iced cake. Spread a thin layer of icing over the top and sides of the cake to seal in crumbs. Refrigerate cake for 30 minutes to harden icing.
Ice the cake with remaining icing on the top and sides. Pull a fork through the icing on the top and sides to create “branches” all over. Using the black writing icing tube, pipe strings for the lights across top of tree. Place assorted colours of sugar-coated chocolates on the black string for the lights.
Press the candy cane pieces at the end of the tree to make the trunk.
Roll out the yellow ready-to-roll icing and using a small star cookie cutter, cut out a star. Place star on top of the tree. Store loosely covered at room temperature.