Auroras are caused by charged particles streaming away from the Sun and towards the Earth. As the particles reach the Earth magnetic field, they spiral along magnetic lines and cause colorful radiation. While the visible light emissions of auroras can easily be seen on Earth, the UV and X-ray emissions are best seen from space, as the Earth’s atmosphere tends to absorb and attenuate these emissions. The colours of the northern lights are determined by a number of factors: 

• the composition of gases in Earth's atmosphere 

• the altitude where the aurora occurs

• the density of the atmosphere 

• the level of energy involved

Green: Green is the most common colour seen from the ground and is produced when charged particles collide with oxygen molecules at altitudes of 100 to 300 km.

Pink and dark red: Occasionally, the lower edge of an aurora will have a pink or dark red fringe, which is produced by nitrogen molecules at altitudes of around 100 km.

Red: A bit higher in the atmosphere (at altitudes of 300 to 400 km), collisions with oxygen atoms produce red auroras.

Blue and purple: Finally, hydrogen and helium molecules can produce blue and purple auroras, but these colours tend to be difficult for our eyes to see against the night sky.

Source: https://www.asc-csa.gc.ca/eng/astronomy/northern-lights/colours-of-northern-lights.asp

This map shows the midnight equatorward boundary of the aurora phenomena at different levels of geomagnetic activity. A Kp=3 represents very low geomagnetic activity, while a Kp=9 represents a very high level. Source: NOAA National Weather Service Space Environment Center