Preheat oven to 180°C (160°C for fan assisted ovens) Gas Mark 4. Line bottom and sides of the11x7-inch rectangular tin with foil and well-grease. Make and bake the brownie mix as directed on the box using water, oil and eggs. Transfer to cooling rack and allow to cool completely; about 1 hour. Freeze in the tin for 30 minutes.
Remove the brownies from the tin using the foil to lift and peel away. Use the sharp knife to cut brownies into 5 rows by 3 rows, making 15 brownie squares (about 5cm x 5cm each).
Spoon the icing evenly between 3 separate small bowls. Stir the icing with the different food colouring gels to your desired shade of colour. Decorate as desired.
To decorate the Polar Bear Brownie: ice the top of the brownie thickly with vanilla icing. Using a fork, scrape the icing to outside of brownie to create spikes of “fur”. Dollop a small mound of vanilla icing in center of the brownie, place 1 brown sugar-coated chocolate on top for the nose, 2 mini brown chocolates for eyes, and 2 white chocolate drops for ears. Pipe a mouth using the black writing icing tube.
To decorate the Gift Brownie: ice the top of the brownie with the green icing. Sprinkle 100's and 1000's on top of iced the brownie. Pipe 2 lines using the red writing icing tube across the top to cross in the middle. Place 5 red sugar-coated chocolates in centre tipped on sides in a circle to form a bow.
To decorate the Santa’s Hat Brownie: ice the top of the brownies with the green icing. Sprinkle the top and edges of brownie with 100's and 1000's. Place 3 red sugar-coated chocolates in a triangle on top of the iced brownie. Cut 2 mini marshmallows in half crosswise. Place 3 of the marshmallow halves for the hat’s brim and one on top of the triangle for pom-pom.
To decorate the Christmas Tree Brownie: ice the top of brownie squares with red icing. Sprinkle the top and edges of the brownie with 100's and 1000's. Place 6 green sugar-coated chocolates in a triangle on top of the iced brownie to form a tree. Place 1 yellow mini chocolate on top of the triangle for star. Place one piece of liquorice at bottom of triangle for tree trunk.
Store covered at room temperature.