Hi and thank you so much for replying! :)

I was starting to lose hope here...

Yes, it happened in waking life. Actually it happened while I was at a job interview.. so you can imagine that I pretended not to notice the change in eye colour...

As for changes in light, this person hasn't moved at all (so the angle of the light did not change) and the intensity of the light in the room remained the same from the start to the end of the interview.

So, I don't think it can be explained by natural phenomena...

I don't know how to explain what happened other than this person may not be human...?

Please let me know if you have any further insights or questions. :)