1.Line 4 oven trays with baking paper.
2.In a medium saucepan, combine ½ cup of the caster sugar and 2 tablespoons of water on low, stirring until sugar has dissolved. Increase heat to medium and simmer, without stirring, for 5 minutes until mixture is reduced and syrupy.
3.Add a few drops blue food colouring and swirl to combine. Add 2 cups popcorn and stir with a wooden spoon until completely covered. Spread popcorn on tray to cool.
4.Repeat 3 more times using red, yellow and green food colouring.
Stirring syrup after boiling causes crystallisation.Note