Grease a 3 cm deep, 16.5cm x 26cm (base) slab pan. Line base and sides with baking paper, allowing a 3 cm overhang at both long ends.
Combine sugar and 2/3 cup hot water in a saucepan over medium heat. Cook, stirring, for 3 minutes or until sugar dissolves and syrup is clear.
Using a whisk or fork, combine gelatine powder and 2/3 cup of cold water in a jug. Pour this mixture into hot syrup. Cook, stirring, for 2 to 3 minutes or until gelatine has dissolved and mixture is clear. Pour into bowl of an electric mixer. Set aside to cool to room temperature (about 30 minutes).
Using an electric mixer, beat sugar and gelatine mixture on high for 6 to 10 minutes or until very thick. Add vanilla and beat for 1 minute.
Spread into prepared pan. Smooth top.
Working quickly, add 10 drops of each food colour across the surface, making sure the red and green drops are not too close together. Using a skewer, create swirl patterns through the food colour (trying not to mix the green and red).
Set aside, at room temperature, for 1 hour or until set. Lift onto a board and cut into squares using a greased or wet knife.