Same problem here, on an i7 iMac /5500 graphics card) that had been through a Recoverye (erased everything on the drive and installed a new MacOS from interet). It was sent to an authorised Apple repair company called Elcare (here in Norway)
Called the company it was bought from (Elkjøp in Norway), and they told that I will receive the iMac in return, because the service company couldn't find anything wrong with it, and said (quote) "If our technicians doesn't find anything wrong with it, there's nothing wrong with it", and someone else from the same company said that "even if you can document that half the screen has a different colour eg. by making a video recording of the screen from you iPhone, we can't help you if the tech engineers can't reproduce the error". This was on a Mac that was paid $3500+ for, and which is covered by 'right of complaint' + is covered by Apple Care and has an insurance (all bought from the same shop) – all this was paid circa $1180 for on top of the Mac price.
In this case, the problem didn't occur only on restart/startup – it popped up out of nothing and didn't go away until I had restated the Mac. There are other problems with the Mac as well.
Have any of you got a permanent fix for the problem?