Brown Eyes Don't Vary

A few months back, my wife of 54 years remarked, with surprise, that I had very blue eyes. I did not know what to make of this belated discovery.

It turns out that people with blue eyes have little melanin in the front layer of their irises. This creates some sort of light-scattering effect that seems blue to the onlooker (there is nothing actually blue in the eye). 

Because lighting and mood changes can alter the size of the pupil, how blue the eyes seems to the observer actually changes from time to time. Somehow my wife had noticed this. Her brown eyes don't vary.