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.
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.