For months I’ve had a problem with my mid 2010 iMac after I upgraded to Yosemite. I would see pink and green/multicolored squares on my screen.
I thought “oh no, here’s a dreaded GPU issue after 4 years on my Mac hardware”. (I wasn’t that surprised, since after five years I had issues with my first gen Macbook Pro’s GPU.) For a while I dealt with my iMac pixel issue, puzzled why it didn’t happen all the time.
A week or so ago I decided to re-format the iMac down to OS 10.9 Mavericks, to see how performance was. Sure enough, the pixel effect went away. Interesting.
More recent Googling turned me to this Apple Support thread. I decided to upgrade back to OS 10.10 and try this fix, and sure enough it worked! Reducing transparency did the trick. I’m guessing there is an issue with older model iMacs and the newer transparency feature of Yosemite. If you run into this issue after upgrading to 10.10 Yosemite, do the following:
Go to System Preferences, the Accessibility:
Then click on “reduce transparency”
I now have my iMac back to awesome working order and couldn’t be happier! Sweeeeet.