Most aurora is green, but on a strong night you may see red, pink or violet too. Each colour is a clue about what's happening 100–300 km overhead.
Here's exactly what makes each colour, and which are the rarest.
Green — the everyday aurora
Green comes from oxygen at ~100–150 km. It's the most common colour because that layer is dense and easily excited. This is the classic curtain most guests see in Ruka.
Red — high and rare
Red comes from oxygen much higher up (200 km+), where the air is thin. It needs strong solar activity and often appears as a faint crimson glow above the green during big storms.
Pink and purple — the edges
The pink and violet fringes at the bottom of active curtains come from nitrogen. They appear during fast, energetic displays and are a sign of a strong, active aurora.
Blue — the rarest
Blue, also from nitrogen, is the hardest to see with the naked eye and usually shows up best in photographs. Cameras capture colours our eyes miss in low light.