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If you’re on a Mac and using a Dell monitor (I know it does it for the Dell Ultrasharp, not sure if it’s for all Dells), check your monitor settings to see if the computer is actually sending RGB to the monitor. Chances are, even over HDMI, it’s using YPbPr. Every mac I’ve ever used (20 or more over the past 10 years), and every Dell monitor (at least 5), the mac is sending the the display signal as YPbPr instead of RGB. Just Google, “Macos Dell YPbPr” and you can find complaints going back 10+ years.

I get a lot of computers to use for a brief period of time (consultant), and fixing that on Macbooks every couple months is non-trivial. Sometimes it’s impossible if it’s an older version of MacOS because older versions required overriding system files and booting into single-user mode.