From my experience using Plasma 6+ and a NVIDIA card, I keep HDR on on my main display (Odyssey Neo G7).

No issues with washed out colours on the desktop, everything looks fine

I can watch HDR videos using the included Haruna player or MPV.

Firefox has no HDR support outside Mac OS, so no HDR on YouTube.

For games, it depends. Some games can detect HDR and work fine, but for most I have to use gamescope, which in itself brings some issues like not having the Steam overlay, games freezing randomly or just having terrible performance due to niceness (everything has a workaround though, but that requires some tinkering). Check my comments about the issues and workarounds

For game scope running HDR, there’s a lot of people and guides telling you to use countless flags that don’t really do anything at all. The best thing to do is to read its documentation. I use the following flags as startup parameters on my Steam games:

gamemoderun gamescope  -W 3840 -H 2160 -r 165 --hdr-enabled --hdr-itm-enable --hdr-itm-sdr-nits 300 -f -e --mangoapp -- %command%

gamemoderun just enables game-mode, which can bring some small performance improvements.

-W -H -r flags are to determine resolution and desired refresh rate. You might be able to omit those flags but I have had some issues with that.

--hdr-enabled is the only flag needed to get HDR working. Nothing else. (except from enabling it on your DE)

--hdr-itm-enable --hdr-itm-sdr-nits are for inverse tone mapping for non HDR games, it’s the same as Windows Auto HDR.

-f is full-screen, but to be fair I don’t think this one is doing anything, but I need to test better.

-e is to enable Steam integration, which should be the overlay and input, but its broken (there’s a workaround, check the last comment made by me there)

--mangoapp is to run mangohud, this flag is preferred over running mangohud before %command%. It’s partially broken this way because it does not dispaly the GPU or gamemode info. Running it as mangohud works 100% fine but apparently there are some issues with it that are beyond my knowledge.