Color management ... more often, mismanagement ... on computers is so frustrating. It's far more complex than any reasonable first glance looks like it should be, it involves many different providers of both apps & services besides the OS, and ... there isn't any over-riding entity that can enforce everyone to do it one straight-forward way.
Cameras mostly produce video with the Rec709 video standard and assume the standard camera-transform function. Then of course, you can have the various log formats, RAW, and CinemaDNG from various makers, all with their own differences. So the incoming is generally X, but with potential Y, Q, P, and N also. Loverly, right?
Computer operating systems have their own internal coding for handling video files. And there's apparently a split currently between the Mac systems (remember, about 10% of total computer user-base) and PC's running Windows. Apparently, from testing done by Adobe's color folks, the current Mac OS's do apply the first part of the Rec709 standard, the camera transform math functions, but not the display transform functions that were added to Rec709 by BT 1886. I'm not sure if the Windows OS fully does, but ... the Mac specifically does not.
Following this so far? Hmmmm ...
Now we can get to the other part of the hardware, the monitors.
Up until the last couple years, all monitors were struggling to even get to the full sRGB color space, let alone cover properly or distinguish the video sRGB color space from standard sRGB. But now, we have wide gamut and even high dynamic range monitors coming in. The two are not the same thing, of course. And they certainly add multiple layers of complication to the whole thing.
Mac has gone all in with the P3 color space, which is a very massive color space compared to sRGB. Something like nearly half the colors you can map to in P3 simply aren't there in sRGB. And it also has a bit different contrast/tonality ... appearance ... than sRGB. Of course, each size of the Mac P3's has a slightly different group profile. Delicious, that ...
What happens when you take a smaller-space file like video sRGB, and 'throw' it onto a huge-space P3 monitor without proper corrections applied by the system and the app displaying the video? Typically, you get less apparent saturation, and because of the tonality changes, the gamma (the "density" curve of the image) is skewed also.
Now ... if the app being used to display the video is aware of both the color space/profile data of the monitor and the colorspace/profile of the video being played, it can attempt to map the video to the monitor in the best way it ... can. PrPro is working in Rec709 ... both transforms ... and ... trying to display a proper image. The new 'enable display color management' option is an attempt to get around the Mac P3 issue and the fact that Mac's don't recognize Bt 1886 display transforms. Which PrPro tries to abide by.
Outside of PrPro, it gets even murkier. Neither the Mac OS nor FCPx apparently utilize the display transform functions of BT1886 as noted above, so ... you can get different "looks" to the file in FCPx and many players on a Mac. Users have fewer options in Macs than typical PC's, so trying to set your Mac around that ain't ... easy.
QuickTime player is of course notoriously color-stuuuupid, makes no attempt to check the flags/tags of the media it plays for color space. Period. Chrome and Safari browsers are similar.
VLC and Potplayer do attempt to handle flags/tags appropriately. As far as they can. But remember ... that assumes they're displaying on an appropriately set system, sRGB/video gamma 2.4. Which on the Macs with P3's, isn't the case.
So ... older Macs & Macbooks (sRGB based) see typically a different view than newer Macs with P3's. Loverly, again, eh?
What do you do?
Well ... broadcast standards are Rec709, and nearly all pro-produced b-cast programs/movies-to-broadcast are produced in that. And BT 1886 is 'controversial', with one of the larger guys in calibration insisting that it can cause over-crushed blacks on a few screens during viewing if you grade with a monitor using both the camera and display transform functions. Having read his detailed piece on that, well ... his objection is that a small percentage of un-managed screens will look slightly worse with the Bt 1886 display transform function in use while grading ... but ignores that the vast majority of screens will see a better image if graded with the Bt 1886 transforms.
So you can argue both ways on the Bt 1886 part, with good company.
I notice that in my paid version of Resolve, while I do have all sorts of delightful options for setting input color space, various transforms for types of media, working space within Resolve, and export color management options ... there's nothing I've seen relating to turning on/off Bt 1886. And in my work between PrPro and Resolve, the program monitors of the two apps show the same image of the same clip. The scopes are identical. Exported out of either one into the other on my system, they match.
I have to assume Resolve must be applying Bt 1886 as part of the Rec 709 options, or that would not be the case.
Resolve, realistically, is the Big Dog of pro colorist grading apps for total users, and probably for users in b-cast work as well.
But for anything in video post to be predictable, to have the ghost of a chance of getting it close on another system it really needs calibrated systems running the same standards. Ergo, calibrating that monitor for video sRGB first ... if the OS will respect that profile for that monitor.
As far as "what my users will see" ... I have found many Mac folk are so insular within their Macosphere they assume that the vast majority of their fellow users of YouTube or whatever over the web will also be using a Mac. Everybody they know does!
Well ... that's not even close to accurate. The vast majority of computers are PCs, there might be nearly as many Linux running rigs as Macs. So designing your material to look somewhat close to as it does on your screen in a non-managed player is designing it for a relatively small subset of potential viewers.
And ... Vimeo tends to do a bit better with color than YouTube, VLC/Potplayer do a lot better than QuickTime player (which many PC's dont' even have installed) ... Firefox is also much better than Chrome/Safari as it does pay attention to color flags/tags of media.
So ... what the heck ... ?
The best advice I've seen and received from colorists is to do as they do ... setup your system as close as possible to b-cast specs, including all calibrations. Use external LUT-driven boxes for your monitors if at all possible, at least your "confidence" monitor. Test your exports on other heavily calibrated systems or especially through the QC machine of a local TV station, if they'll test a file for you (many will). See if it accepts or rejects your file, and those machines use tough standards. A few pixels over max allowed saturation or DR, it's dumped as not complying.
YouTube ... what a nightmare. Typically it accepts H.264 uploads t-coding them into a temp codec, then re-encodes them a bit later ... supposedly. For me, that works, and what I've uploaded looks the same in YouTube over Firefox on my confidence monitor. For probably the majority of users, for some reason, that second encode doesn't happen. You can force it, by going into your YouTube channel after uploading, select the vid, go to the retouch/modify section, don't even do anything to change the brightness or whatnot, just save. It will then re-encode and your vid should be in the proper color space recognition.
Or ... Averdahl, another very experienced user here, recommends just using the Cineform 10-bit codec for exporting from PrPro to upload to YouTube. It will take longer to do the upload, but ... you get around the mis-color-tag problem. Jim Simons I think suggests DNxHD/R, and that also is going to be bigger but avoid the color tag issue.
Out in "The Wild" ... just understand you have no control. You material will be watched by all sorts of phones in all sorts of environments from bright daylight to dark rooms. Computers from offices brightly lit to semi-darkened rooms, with all sorts of color spaces, most of them not nearly close to calibrated.
So ... what do you do? It's back to the colorist advice ... send out as close as possible to b-cast standards, and give it up.
At least, done this way, your materiel will look relatively the same as all other pro b-cast quality work on that screen. No, it will never look like it does on yours. Just give it the best chance to look professional in relation to other professionally produced work.