I have actually successfully fixed a 2010 27" imac that had this discoloration in one corner problem. I found the solution and detailed instructions online.
I did a quick search now, and I think it might have been here-abouts:
Repair guides and support for 27" Intel iMacs by Apple, introduced October 2009.
www.ifixit.com
...but I'm not shure, though. I'll have to fire up my old mac pro or imac and see if I saved the link, and I'm unable to do that right now.
It was a question of taking the imac apart and fixing a tiny wire to reconnect it to the screen. I remember being able to actually just press hard on the corner of the display and it temprarily worked until I let go.
I'll get back to you if I can relocate the exact instructions.
That would be great.
To add to everyone’s stories:
I bought a 2010 27-inch iMac Built-to-Order with my preferred specs. Arrived and had obvious yellow in the corners, maybe 20% of the screen cumulatively. I’m a graphic designer so not only am I visually sensitive to color inconsistency, but color inconsistency will affect my work and make referencing colors inaccurate.
So I called Apple Customer Support, who were great! And had my iMac replaced in the mail. Took a week or two. Opened the box, same issue. Called again, had it returned, replaced, a week later, same issue. Every panel had the same yellow discoloration.
At that point I needed to get work done and couldn’t Afford to wait around weeks at a time. I just sucked it up.
Eventually I got less bothered by it, but it never went away, and it was obvious when the screen was mostly white, like viewing web pages full screen.
It permanently turned me off to the idea of iMacs. I would prefer a Mac Pro for regular consumers so that we could buy our own screen.
I don’t understand why Apple doesn’t just release an iMac, component wise, but without the screen. I need a dedicated GPU, and the only option starts at $5,000 for a Mac Pro. It’s overkill for graphic design. A $2,000 machine is overkill for graphic design. Why can’t Apple just make a Mac Mini version with dGPU ffs?