Heat the oil in a heavy-based pan over a medium heat and stir the popcorn kernels into the oil. Cover, reduce the heat to medium-low, and shake the pan every so often until you no longer hear any popping, about 4-6 mins. Remove from the heat and uncover just enough to release the steam, but be careful not to remove the lid completely in case any kernels continue to pop.
Dissolve half the sugar in a medium pan with 2 tsp water, then add a few drops of the red food colouring and stir well. Remove from the heat, add half the popcorn, and stir until well-coated in the red sugar syrup. Tip onto a baking tray lined with baking parchment, then leave to dry. Repeat with the remaining sugar, the green food colouring and the rest of the popcorn. Leave both to dry completely, about 3-4 hrs (this stops the food colouring from staining anything once hung up).
Thread the cotton through the needle to the length you want the bunting to be, doubling the thread up. Cut, and tie the two ends in a knot. Push the needle through the thickest part of the popcorn and ease the popcorn down the thread, alternating the colours. Or, make two separate buntings. Knot the end to secure, then use as a decoration.