The unique colors of light produced by a gas are called its "spectrum". The auroral lights' colors are determined by the spectra
of gases in the Earth's atmosphere, and the height at which the most collisions take place. Incoming particles tend to collide with different gases at different heights.
Very high in the ionosphere above 80 miles, oxygen is the most common atom, and collisions there can create a rare red aurora.
The strong yellow-to-green light that is most common is produced by collisions with oxygen at lower altitudes, between 100 and 300 km.
Around 100 km, nitrogen molecules produce a red light that often seems to form the lower fringes on aural curtains.
If our atmosphere were neon instead of oxygen and nitrogen, what color would auroral lights be? You guessed it, orange!