The Avada Kedavra curse is recognisable by the flash of green light and the rushing noise emitted from the caster's wand.
- Harry Potter wiki, Killing Curse article, Nature Section
I thought green was commonly one of the colors used by light side (the good side). Like in Star Wars, where Jedi light sabers are green or blue, but Sith light sabers are mostly red. But in Harry Potter the light emitted after casting the Killing Curse is green and the defensive spell (casted by Harry Potter) is red. Why the colors are switched in Harry Potter?