If the accompanying photo does not make sense to you, it will after you visit this page http://michaelbach.de/ot/col-context/ in my website on     #visualperception , visual phenomena and   #opticalillusion .

Hint: it's about the differences in eye colour.

I'm surprised myself on the strength of this effect, even with little opacity of the red overlay. This brings the count of my collection on visual phenomena to 109 items :).