Do any of your iMac’s have “Grey Smudge Marks” on the screen? Pervious versions (2009-2010) of iMac have had the “yellow tinge” issue..
is it like a sort of grayish sort of dotted smudged spot, like you pressed it with the palm of your hand.
if so, its not covered by warranty you will probably be charged for cleaning the backside of the screen..thats my experience…
I know of someone who took of the screen himself (pretty easy to do) and cleaned it with a € 20,- Monster Mac screen clean spray for about 3/4 times really good and it fixed the problem.
Yes it’s like smudge marks, but, it cant be cleaned.
i know what kind of staines you have/mean, try contacting apple support because this is a common problem. It’s usually caused by humidity (in the air) that gets trapped between the actual display and the glass..that’s what i got told.
You are not the first person i know to have this problem…so, Good luck.