Well the first thing to mention is yes. Auras change almost continually given so many different sorts of occasions, stimuli, moods etc, they're by no means a static thing. Secondly though, and in my opinion far more importantly, seeing auras is an interpretation of energy. For that reason everyone has there own opinion of what these specific energies are and see colors very different to any other individual.
Given all you said however, I'm not even convinced they were talking about an aura. The color of an aura really shouldn't persuade you of their personality like that and even if it did I highly doubt any serious practitioner would just announce it in the street. At worst I'd say that the couple were just trying to mess with his head for fun, otherwise I'd probably put it down to a slight mental health issue and leave it at that.