The aurora is caused by the interaction of high-energy particles (usually electrons) with neutral atoms in earth’s upper atmosphere. The colour of the aurora depends on the wavelength of the light emitted. This is determined by the specific atmospheric gas and its electrical state, and the energy of the particle that hits the atmospheric gas. The atmosphere consists mainly of nitrogen and oxygen, which emit the characteristic colours of their respective line spectra. Atomic oxygen is responsible for the two main colours of green (wavelength of 557.7 nm) and red (630.0 nm). Nitrogen causes blue and deep red hues.
Most of the auroral features are greenish-yellow, but sometimes the tall rays will turn red at their tops and along their lower edges. On rare occasions, sunlight will hit the top part of the auroral rays to create a faint blue colour.
Eventhough I've shared this image before, I thought I'd share it again as it is a great example of what has been explained above and clearly shows pretty much the entire spectrum of colours 
NOTE: This image was taken at a KP7 and it showed the entire gamut of colours very briefly. To see all the images taken on the night, click here.
