Do you mean the game beta or OSX? The red tint issue should be fixed with the current 10.14.4 beta 2.
Does that iMac have HDR support? A quick search online implies that it doesn't (EDIT: but it does; maybe the confusion was the bug mentioned below when applying the settings?). In any case, the darkness issue is a symptom of enabling HDR with an OSX system that doesn't support it, so if your iMac has HDR support, the game is not detecting that for some reason.
I still have what is considered the top-of-the-line consumer iMac. It's basically the best iMac you can get other than the iMac Pros. When the Mac version of Divinity Original Sin 2 was first announced I checked on the HDR thing and found that my iMac does support that.
I have a dual monitor setup here with the second display being a 4K LG 2018 TV that also supports HDR. With the HDR option on I tried lanching the game on each display, with each display set as the default display, and encountered the same problem on both.
Once again I'm running OS X Mojave 10.14.3, which is the latest full release. I'm not really comfortable with downloading a beta so all have to wait until 10.14.4 is officially released to try that.
If I get a chance I'll try launching the game again today with HDR on and play around a little more with that gamma slider to see if I can get better results. But I'm guessing I may have to wait until 10.14.4 is released for this to function correctly?
Thanks for your help.