Recently, I wanted to buy a second hand 27" iMac. I learned that there are issues with the screen showing a "yellow tint (tinge)". There were tests available at tapplox.com but these links no longer work. I recreated the tests from a snapshot of that website on the Internet Archive and made them available here. Please read the test instructions below or watch the instruction video before testing your screen.
Source: forums.macrumors.com
Go to the link below with the height and width of your browser window maximized.
Note: Your screen may have the yellow tinge on a corner as opposed to the entire bottom. The following links might be useful to help you look for them.