Call Apple to report this issue so they are aware of the problem. Even if they can't offer a solution right away, at least they have a record of you having trouble and can incrememnt the record they have of the number of people experiencing this problem.
How to Force RGB Color Output instead of YPbPr on your M1 Apple Silicon Mac for an External Monitor.
This step-by-step video tutorial will guide you through the procedure of forcing RGB color output on your M1 Mac.
Here is the direct link to the video tutorial: https://www.youtube.com/watch?v=Z1EqH3fd0V4
The video also has Closed Captions (Subtitles) that you can enable, to make it easier to follow if needed.
Please note that you're doing any changes on your own risk.
Here are each of the Terminal commands mentioned in the tutorial, so that you can just copy and paste them:
open /Library/Preferences
plutil -convert xml1
plutil -convert binary1
plutil -lint
The end result is having your M1 mac output RGB color to your external monitor instead of YPbPr,
potentially making the colors more accurate and the text a bit more crisp, even on older 1080p monitors.
Hopefully this tutorial would be useful to someone.
Please feel free to ask in the comment section if you have any questions regarding this procedure.
While doing a lot of testing on how the Dual-Cable workaround makes RGB to work on M1, I've discovered what changes it makes to macOS, and managed to create a more streamlined workaround without the need to use a second cable.
To make things easier, I've created a step-by-step video tutorial of the whole procedure that should force RGB color output on your M1 Mac connected to an external monitor, and works on an HDMI-to-HDMI cable connection.
Big thanks goes to the amazing community and all their help over the years to solve issues like this:
https://gist.github.com/ejdyksen/8302862
https://gist.github.com/adaugherity/7435890
Apple Open Source Project Files for Displays and Graphics
https://opensource.apple.com/source/IOKitUser/IOKitUser-1445.60.1/graphics.subproj/IODisplayLib.c
https://opensource.apple.com/source/IOGraphics/IOGraphics-517.17/IOGraphicsFamily/IOFramebuffer.cpp.auto.html
How to Edit and Convert binary plist files
http://hints.macworld.com/article.php?story=20050803111126899
https://apple.stackexchange.com/questions/155393/how-to-beautify-binary-dict-files
https://discussions.apple.com/thread/1768480
@sudowork has created an awesome script written in Phyton that automates the steps and checks for duplicate files.
You can find more info about it here:
https://github.com/sudowork/fix_m1_rgb
Multiple PixelEncoding and Range keys in the same plist file
Note that there might be multiple instances of the PixelEncoding and Range keys in the same file, one for each output of your monitor and for different AirPlay devices. You might need to update the integer on each one to get RGB color output on all displays.
Getting RGB color only before login
There might be multiple duplicate plist files with the same name in different locations.
Make sure that you only have the main modified file in:
/Library/Preferences
Then make a backup and remove duplicate displays plist files from these locations (if any):
~/Library/Preferences
or
/Users/username/Library/Preferences
and
/Users/username/Library/Preferences/ByHost
USB-C to DisplayPort
From what I've seen, it seems that macOS Monterey 12.0.1 finally outputs RGB color by default on some monitors when using USB-C to DisplayPort cable on M1 Apple Silicone Macs.
You might need to make a backup and delete these 2 files:
/Library/Preferences/com.apple.windowserver.displays.plist
and
/Users/yourname/Library/Preferences/ByHost/com.apple.windowserver.displays.[UDID].plist
Restart your Mac and it should properly output RGB color on the monitor on the next boot.
HDMI to HDMI
The situation with HDMI seems to got a bit more complicated. Now the whole section for the LinkDescription might be missing from the com.apple.windowserver.displays.plist on a clean install and doesn't seem to be recreated when rotating the screen either.
Luckily, the solution still works, but you might need to manually add this whole section in the displays plist file:
<key>LinkDescription</key>
<dict>
<key>BitDepth</key>
<integer>8</integer>
<key>EOTF</key>
<integer>0</integer>
<key>PixelEncoding</key>
<integer>0</integer>
<key>Range</key>
<integer>1</integer>
</dict>
The section usually goes right under the CurrentInfo key, and it should look something like this:
This should get your RGB color output working on M1 Mac mini, even when connected with HDMI to HDMI cable.
Multiple monitors when one them is using HDMI to HDMI
Additional thanks goes to @somogyi-ede who tested this with multiple monitors and confirmed that the LinkDescription key needs to be added under each monitor instance in order for all of them to receive RGB color output. Link to the comment
If you have any additional questions, please feel free to contact me.