iMac mid 2011 grey screen and colour bars

Have you blown the dust out lately?

iMacs are very good dust collectors, undercooled, & prone to GPU failures.

See if you can get this & ramp the fans up...

https://www.crystalidea.com/macs-fan-control

That model 27" iMac had a graphic card recall a while back (it's way past now). The card was AMD Radeon HD 6970M.

If that's your iMac and graphic card model then it is a likely suspect. Not much you can do for that except take it into an Apple Genius bar for a free diagnostic or to an Authorized Apple Repair Center.

That is a common problem with Sierra & at least 5K Macs, some have fixed it with...

I have had some luck selecting alternate scaled displays. My iMac does not show the horrible pixels upon shutdown. The problem is, I do not like any of the alternate scaled displays. They are either too large or too small.

When I select DEFAULT under scaled display, it reverts back to the unscaled default display. And then the shutdown pixel problem happens again.

multi colored screen when shutting down iMac

Please Brazilian kill my mac it happens every time a new update is instaled on diferents imac. dont tell is a hardware problem because we know the probrem was a bug on drivers. I want my apple back. Im getting legal advice and i will go to course. Thats is a end on a love storie that i had with mac since 1998. Lets get divorce than... im a porr guy from brazil who work for years to by a mac to become a dj and now my dreams is over. Mac os x do not provide any support or drive for my numark and m-audio so i need tobuy new dj controller and audio card on top of that a new mac do you know how much it cost in brazil? please help im um a unemployed Brazilian dealing with a heavy ecomical strugle. And all i need from you is a peace o mind and my computer worlking so i can get back with my music creation;

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I have a very similar problem, not quite the same as yours. I have a brand new iMac, 27" 5K Retina Late 2015. Every now and then, I get horizontal colored bars when I shut down. Instead of the screen being very dark gray just before it goes black when shut down, I get the colored bars, then it goes black after a few seconds. I reported it to Apple tech support, and the technician said he was going to talk to engineering. I haven't heard back yet.

I just bought my iMac about a month ago. According to Apple's site, the 27" iMac with the 1 TB fusion drive, which is my model, has the "AMD Radeon R9 M390 graphics processor with 2GB of GDDR5 memory". That's what it's being sold with now. I'm assuming for the moment that's what mine has, since I just very recently bought it, but I'm away from my iMac now. I will check later as to what's actually on it.

Your issue is different since your iMac model is different. I suggest you start a new topic, specify the model of the Mac and graphic card and post details of the problem. That way you'll get responses specifically for your issue.