NASA:
Many aurora appear green, but sometimes — as in this image from the International Space Station — other colors such as red can appear. The colors depend on which atoms are causing the splash of light seen in the aurora. In most cases, the light comes when a charged particle sweeps in from the solar wind and collides with an oxygen atom in Earth’s atmosphere. This produces a green photon, so most aurora appear green. However, lower-energy oxygen collisions as well as collisions with nitrogen atoms can produce red photons -- so sometimes aurora also show a red band as seen here.
