There are specific gases that produce a white discharge, eg. Argon. But in this case I suspect that it is imperfectly evacuated air (it needs to be imperfect, otherwise it would be virtually impossible to get a Geissler tube to discharge).
A natural atmospheric combination of mostly Nitrogen (pinkish white to bluish white depending on current density), Oxygen (a bit of Violet/Lavender), CO2 (again, bluish white to pink) and other trace gasses do combine to something whitish at the right pressure. The Automatic White Balance of your camera is probably influencing what we see versus what you see too.
As I mentioned previously, internal pressure was a fairly critical 'trial and error' factor in getting early Geissler tubes to produce a decent light.... and that's a decent light!