Agreed, on the late 2015 iMacs, I still think its graphics related, although, I had everything replaced on my other late 2015 i5.
I have just looked up the cost for a new iMac with the same spec as my i7 - £3,638.
Although they are offering £495 for my Late 2015 27" i7 4Ghz, 32GB ram, 512ssd - I'm was tempted and then saw they have a refurb i7 - same date and spec but with 16GB on sale for £2,700!
I would probably get around £1700 for mine on the market, but wouldn't feel right passing it on as it is, I'm now living with the problems, as I have since late 2015.
I have ongoing glitches (most days), a restart normally sorts it out.
What's puzzling to me is the list of problems I'm having, it's always changing, I find as one glitch fixes itself, something new will happen.
The startup and shutdown artefact glitch fixed itself for a few months, then appeared back with HS.
It hasn't happened for quite a few days now.
But, I am having this today, and it's a brand new glitch, for me anyway.
Glad to hear you got everything sorted and your 2017 iMac is good, you never know, I might get tempted, but can't see it yet.
Obviously it should be red (as bottom) but after a short sleep the colours shifted, only a file save and PS restart would fix it.
Just as a final note, I have noticed the screen colours a lot darker, just thought I would mention.
The thread has been quiet so hopefully its just me!