Why is the color different based on Altitude? Short answer: different light emitting oxygen species take different amounts of time to do so, and at lower altitudes the one that emits red can't survive long enough to emit a photon on its own before bumping into something else (atmosphere is denser the lower you get), which rob it of the energy needed to emit light.
Straight from the NOAA Space Weather Prediction Center's
"Aurora Tutorial" page:
Different auroral colors come from different heights in the atmosphere primarily because the life-time of an excited atom or molecule (time spent in its excited state) is vastly different for different colors of the aurora. The green aurora from oxygen in the 1S state typically occurs from 120 to 400 km (80 to 250 miles) above the surface of Earth. The red aurora from oxygen in the 1D state is restricted to altitudes above 300 km (180 km). This is because oxygen in the 1D state has a very long lifetime (>150 sec) and can only survive in the thinner atmosphere above 300 km. At lower altitudes the oxygen in the 1D state collides with other atmospheric atoms or molecules before it can emit a photon which deactivates or quenches the excited oxygen. The 1S state of oxygen has a lifetime of about 1 second and therefore emits a photon more quickly and thus can emit at lower altitudes where the density is higher. The aurora sometimes has a purplish lower border which comes from emissions from molecular nitrogen. This "prompt" emission is emitted from excited states of nitrogen that have almost no delay between excitation and emission. It survives at even lower altitudes between 120 and 200 km (80 to 120 miles). In the video sequence at the end of this readme document, the purple emissions from the nitrogen seems to lead the green emissions from the oxygen. This is an example of the difference between the prompt emission of nitrogen vs the emission from the longer-lived excited state of oxygen.I recommend visiting the page. It's a pretty good primer on the aurora.