Since 2009 I have went through 3.or 4 iMacs It was making me that bad I had to get an Advocate to take over. There is a site called DirtyScreens.com
The three faults are 1. Yellow Tinging 2. Flickering screen 3. Dark and patchy marks.
This problem is still ongoing. The Advocate got my refund of £1230 off Apple. Apple tried to shut the dirty screen site down but it was in France. The guy gave me a full list mounting to over a thousand people in France,and that`s not counting the rest of the world. I was going to buy the new Mac Mini but that`s faulted has well,but Apple and Intel have owned up to this one. Google your problem type in iMac Faulty Screen you be amazed what comes up. I have evan got a protest video,and about a dozen showing you the faulty screen. I live in England if they can`t fix it they can offer you a new one,but if that get`s the same fault you are entitled to your money back. I wish you luck.