As the electrons enter the earth's upper atmosphere, they will encounter atoms of oxygen and nitrogen at altitudes from 20 to 200 miles above the earth's surface. The color of the aurora depends on which atom is struck, and the altitude of the meeting.
Green - oxygen, up to 150 miles in altitude
Red - oxygen, above 150 miles in altitude
Blue - nitrogen, up to 60 miles in altitude
Purple/violet - nitrogen, above 60 miles in altitude
All of the magnetic and electrical forces react with one another in constantly shifting combinations. These shifts and flows can be seen as the auroras "dance," moving along with the atmospheric currents that can reach 20,000,000 amperes at 50,000 volts. (In contrast, the circuit breakers in your home will disengage when current flow exceeds 15-30 amperes at 120 volts.)