Witches and wizards often reveal themselves to each other in public by wearing purple or green, often in combination. In Britain purple has an association with both royalty and religion. (…) Green has long had a supernatural connection in the UK. (…) Green is the colour of much ‘Dark’ magic; of the 'Dark Mark’, of the luminescent potion in which Voldemort conceals one of his Horcruxes, of many 'Dark’ spells and curses, and of Slytherin house.
The combination of purple and green, therefore, is suggestive of both sides of magic: the noble and the ignoble, the helpful and the destructive.