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The more I examine my screen the more I think the viewing angle is causing most if not all of the off-white/yellow I am seeing at the bottom. I downloaded a colorimeter app (free app called "Colorometer") to my iPhone and measured my screen, and it measured no difference from top top bottom.

I've used this app too but got wildly different readings from top to bottom. Even measuring the same place gives me different readings. Not sure if the app is just that inaccurate or if I'm somehow using it incorrectly.
 
Ok, I went to yet another Apple Store today and checked out a number of the iMacs they had on display. This is one of the bigger stores so I didn't hit all of them. Anyway, each one I checked out had the same yellow tint issue. Some where more obvious than others but I took this to be more from the different lighting conditions of the different stations in the store. Even the Thunderbolt Displays showed this. So this must be just the way they are.

I also took a trip downstairs to the Microsoft Store to check out the AIO computers they had. There was a Dell, a Lenovo and an HP. None of them show this color issue. My guess is that they are either using different manufactured screens or they are just plain better (I know… shocking).

One last thought I had was to bring my iMac into the store and have someone from the Genius Bar check it out. It would be good to get another set of eyes on it. Of course, it weighs a ton and it would be a royal pain to lug it over there. Has anyone done this?
 
Ok, I went to yet another Apple Store today and checked out a number of the iMacs they had on display. This is one of the bigger stores so I didn't hit all of them. Anyway, each one I checked out had the same yellow tint issue. Some where more obvious than others but I took this to be more from the different lighting conditions of the different stations in the store. Even the Thunderbolt Displays showed this. So this must be just the way they are.

I also took a trip downstairs to the Microsoft Store to check out the AIO computers they had. There was a Dell, a Lenovo and an HP. None of them show this color issue. My guess is that they are either using different manufactured screens or they are just plain better (I know… shocking).

One last thought I had was to bring my iMac into the store and have someone from the Genius Bar check it out. It would be good to get another set of eyes on it. Of course, it weighs a ton and it would be a royal pain to lug it over there. Has anyone done this?

Yes. I had one genius state they could and one, during the same appointment, state they had no idea what I was talking about. Results may vary.
 
Ok, I went to yet another Apple Store today and checked out a number of the iMacs they had on display. This is one of the bigger stores so I didn't hit all of them. Anyway, each one I checked out had the same yellow tint issue. Some where more obvious than others but I took this to be more from the different lighting conditions of the different stations in the store. Even the Thunderbolt Displays showed this. So this must be just the way they are.

I also took a trip downstairs to the Microsoft Store to check out the AIO computers they had. There was a Dell, a Lenovo and an HP. None of them show this color issue. My guess is that they are either using different manufactured screens or they are just plain better (I know… shocking).

One last thought I had was to bring my iMac into the store and have someone from the Genius Bar check it out. It would be good to get another set of eyes on it. Of course, it weighs a ton and it would be a royal pain to lug it over there. Has anyone done this?

This yellow that you see, that's looking at it straight-on?

I have a refurbed late '12 27" and it looks very uniform with a white background. Maybe I detect a touch of yellowish right at the screen border.
 
Do you still have your 2009 iMac? If you do, would you be willing to take some side by side photos of it vs. your 2013 iMac with a white background? I'm trying to figure out if I really have a "yellow screen issue" or if I'm just being too picky about shade and that difference I see top to bottom on my 2013 iMac is normal.

Thanks!

I don't have the 2009 anymore. It is easy to tell if you have a "yellow screen issue". The yellow on my 2009 was an irregular shaped blot of yellow that covered the bottom right third of the screen. It looked just like a stain would look like if yellow liquid had seeped under the glass.
 
This yellow that you see, that's looking at it straight-on?

I have a refurbed late '12 27" and it looks very uniform with a white background. Maybe I detect a touch of yellowish right at the screen border.

I posted a photo of what I'm talking about in another thread about the same issue. You can check it out here:

https://forums.macrumors.com/showthread.php?p=18354343#post18354343

Notice how the background of the finder window at the top of the screen is normal but the one on the bottom part of the screen has a yellow tinge to it (some have called this an off-white color).
 
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