The Aurora Borealis, or Northern Lights, are caused by energetic and polarized particles of plasma from a solar flare being caught by Earth's magnetic field and directed towards the polar regions, where the magnetic field dips down through our atmosphere. As the energetic particles interact with the gasses in the atmosphere they can cause a brilliant light display, like pulsing neon lights, giving a spectacular show. But they are seen as bad omens to the Eskimos, and the Muggles watch out for them because if the effect is too strong it can disrupt their electrical grids and knock out the satellites they rely on for communication and navigation. To the ancient Romans, Aurora was the goddess of the dawn, but was not specifically associated with the Northern Lights. To the Australian Aboriginies, the Aurora Australis is associated with fire, and sometimes as an evil spirit creating fire. Some Native Americans saw the lights as the spirits of their departed friends dancing in the sky, which could be a good sign.
Green is a very common color for auroras, and that comes from interaction with oxygen atoms at lower levels, while at higher altitudes oxygen will produce red auroras. Nitrogen normally produces blue, and sometimes red. Other gasses can add to the colors.