Background color issue, Chrome on Retina iMac. Hardware acceleration related?
Unfortunately this is an old known issue: https://bugs.chromium.org/p/chromium/issues/detail?id=44872 which is still not solved.
My suggestion is to try replicate the background-color by using an image as a background.
I have been in the same place before a while and this is how I solved it.
I'm no expert on these kind of issues, but when the 5k retina iMac was issued with a tech-spec, there was alot of fuss and huss around the usage of P3 color gamut.
Specifically to the issue you are having, as you can see from several different sources, some adjustments need to be done before your website will be able to show properly on the retina screen. On Chrome, BTW, it's still an open issue.
To finish off, I got to say I haven't seen a best-practice to this issue, except from actually buying two screens, and manually looking at each picture/website.
Hope this helps, even for a starting point to this strange issue...
Further Reading: