My observations were made assuming that the cabin depressurised from its normal value of around 7-8000ft, to ambient ( anything above 27,000ft, let's say ) so the dewpoint would be reached and a mist would appear.
With reference to the Helois crash near Athens: That aircraft had flown unpressurised at altitude for well over an hour. Wasn't it reported that many of the recovered bodies were very cold, or frozen?