Why do auroras appear in different colors and how do they get that way?
The different colors of the aurora arise because different auroral particles are exciting different chemical species (mainly atomic oxygen and molecular nitrogen) are being bombarded by auroral particles. Depending on how energetic these particles are, different energy-level transitions are being excited by the bombardment. Green light is common, coming from atomic oxygen. If the particles are very energetic, red and blue light may start appearing near the bottom of the auroral curtains.
