Basically, the image looks fine in premiere but loses saturation and contrast on export.
I'm exporting H264 25p on an iMac version 10.12.6 and the Premiere CC 2017
Tried different bitrates and even Quicktime exports, makes no difference.
THE WEIRD THING IS: It looks bad on every program other than Premiere, but when I import the exported video back into premiere it looks exactly the same as the original sequence! It's as if premiere has its own calibrated monitor...
Pleease help, this is driving me crazy!
I think Jamie LeJeune has it right in the following thread from the Blackmagic forums. He's specifically talking about Resolve, but the idea holds true for all NLEs. The upshot is, "The only image you can trust is to run SDI out to an accurately calibrated reference monitor."
Your monitor is not a proper monitor for pro video standards. That monitor is probably in P3 profile, VERY different from the Rec709 video standard which is sRGB.
PrPro will attempt to hold everything to Rec709/sRGB internally and in its monitors. But outside PrPro your system won't show it correctly.
Hence Jim's comments.
If you got a spendy Flanders Scientific b-cast monitor it would come set up for sRGB and Rec709. For a good reason.
Or for less money, you can get by with a foo quality monitor that is natively sRGB, covering 99% or better of that space, with a puck & software calibration system. But make sure you send sequences to be tested by someone with full b-cast gear for real use testing.