As stated before, auroral particles with higher energies deposit more of their energy at lower altitudes where they encounter more collisions with the air. Lower energetic particles don't get that far. The dominant red and green emissions of the aurora are basic excitions of the $\ce{O}$ atom. The red emission at around 630 nm peaks at above 200 km. altitude. It's caused by the excited state $\ce{O}^1D$ with an energy of $\sim5.6$ eV. The green emission at around 558 nm peaks at around 110 km, considerably lower than the red emission. The corresponding state $\ce{O}^1S$ has a higher excitation energy of around $10$ eV. Peaking even lower at around 90 km altitude, the violet $\ce{N2+} $ emission has a wavelength around 428 nm and an excitation energy of around $100$ eV.

The combination of the concentrations of the chemical species and, most importantly, the altitude dependent energy deposition result in the altitude dependence of the colours of the aurora. The further to the violet the light, the higher its energy per photon.

Note that these discussions about what colour is caused by what atom or molecule at what altitude are somewhat simplified. This all only refers to the dominant emission at a certain peak altitude which may overpower fainter colours. To a certain degree, most colours will be emitted from most altitudes, but not be visible to the ground observer. And there are many more colours emitted than the ones discussed.