Jim, I’ve had the same issue since I bought my 12-core iMac Pro several months ago. I’ve spent the entire morning troubleshooting, and your advice has helped a lot. I opened up the same projects on my previous computer (Macbook Pro with Cinema Display) and the differences pretty much vanished. That’s how I knew it was an issue with my new computer’s display settings.
In the end, the default display profile (simply called “iMac”) was mostly to blame. I ended up using “sRGB IEC61966-2.1” instead, and that got me most of the way. Then I followed your advice in creating a custom adjustment layer (except I used Lumetri instead of Colorista) that I slap on the entire sequence right before I export. It took me HOURS of exporting little snippets and then comparing the exported film with the “non-adjustment-layered” sequence to get it to match. Still not perfect but a heckuva lot closer.
I agree that skin tones are where it was most obvious. Really frustrating to get a brand new fancy computer and have the end product look crappier than a few months earlier.
Thanks, Jim, for struggling through the process for the rest of us, and sorry you had to deal with some really arrogant idiots along the way.