macOS incorrect colours on external monitor

I bought a new OLED monitor, which worked great with my Windows machine, but when I connected it to my MacBook Air and MacBook Pro via a USB-C to HDMI* adapter the colours all looked very wrong.

Blacks were tinted pink/fuchsia/magenta, whites were tinted yellow/green, all colours look wrong, yet the macOS UI is still legible. Turns out the issue is that macOS thinks the monitor is a TV and encodes colours as YPbPr instead of RGB.

photo of Big Sur wallpaper with bad colour encoding
How the Big Sur wallpaper looked
photo of colour gamut with bad colour encoding
The colour information is all there, just… wrong.

There appear to be two fixes:

  1. For Intel macs, overriding the EDID information via a plist at /Library/Displays/Contents/Resources/Overrides/DisplayVendorID-$VENDORID/DisplayProductID-$PRODUCTID. There’s a handy patch-edid.rb script for this purpose. It works by setting the IODisplayEDID key in the plist with a base64-encoded EDID payload. While the script clobbers other potentially useful EDID information, it’s a useful starting point.
  2. For Apple Silicon, overriding the LinkDescription in /Library/Preferences/ See Force-RGB-color-on-M1-Mac for a great description of this method. I haven’t tested it though, because I only have Intel macs.

Figuring out the fix was a bit of a process for me: my first few searches suggested it could be a cable problem, but I verified the cables worked just fine with the same monitor and other computers. It also didn’t look like the typical colour aberrations when you’re just “missing” a colour channel. Then I thought it was a problem with the USB-C to HDMI adapter I was using. When I stumbled on old (2013) blog and forum posts I was initially skeptical that it was the same issue or that the workarounds would still work. But it turns out that it is a macOS bug that’s been around for 10+ years, and the old workarounds still work!

* I know DisplayPort would be better, but I want to leave the monitor’s DisplayPort socket permanently plugged into my PC.

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