Colour comes from altitude: oxygen glows green at 100-200 km (557.7 nm, the wavelength eyes are most sensitive to), oxygen glows deep red above ~200 km where low density slows the emission, and nitrogen adds blue/magenta fringes below ~100 km — only reached when particles are energetic enough, i.e. during strong storms.
oval colatitude (deg) = 15 + 3.9 x Kp
That is a rough fit to the real relationship: Kp 0 keeps the oval within ~15° of the pole (high-Arctic only); each extra Kp step pushes its equatorward edge a few degrees further out; by Kp 9 the edge can reach 40-50° from the pole.
- Kp index expands the ring outward, brightens it, and — past Kp 4 — reveals a faint red cap above the green band, then — past Kp 6 — a blue/magenta fringe below it.
- Observer location is compared against the ring's current edge: you can see the aurora once the oval has expanded out to (or past) your geomagnetic latitude.
- Altitude separation between the three colour bands is exaggerated here (real ones are only ~100 km apart, under 2% of Earth's radius) so all three stay visible at once.