49" EU KS8000 user here, mostly using my Switch & PS4 Pro on it, latest firmware as far as I know, 'HDMI UHD Color' only being activated on HDMI 1 where the PS4 Pro is connected.
Something really weird happened to me two days ago, so I thought it'd be interesting to share it here.
I decided to watch some 4K HDR videos again on the native YouTube app, no problem, looked amazing as usual.
Then I turned my Switch on and almost immediately noticed something different with the colors; they looked unusually vibrant, quite oversaturated actually with some of them even looking off.
I didn't care much and played Pokémon Let's Go Pikachu without issue, but even ingame some colors were really popping and I thought it could be me, like I saw them differently for some reason.
Yesterday I played my Switch again (though I didn't watch any HDR videos beforehand) the everything looked the same, to the point it actually bothered me so I went into my picture settings.
No setting was changed, Color Space was still on Auto, so I reseted the settings to default (while still in Game mode of course) and set them as they were previously.
As I expected, it didn't change anything; that weird 'post HDR oversaturated colors mode' was still activated for some reason.
I then did a simple thing just to see if that could make the picture normal again: I changed the source to a currently unused HDMI (HDMI 2 - DVI), then launched YouTube and HDR videos from there.
I quitted the YouTube app, then came back to the Switch's HDMI source (HDMI 3). Guess what, everything went back to normal!
I haven't tried with the native Netflix app (I don't have any active HDR-compatible subscription), and I'm pretty sure it only happens after watching HDR videos on native apps.
So I'm glad I quickly found a solution, but I was wondering if anyone else noticed that? And what could be the reason it happens?
What's really weird to me is that HDR is supposed to be turn off automatically when leaving any HDR video playback (at least on YouTube), and even if that weren't the case for some reason, native apps are supposed to use their own 'source' and not mess with the current one settings-wise.
tl;dr => it seems watching an HDR video on native apps directly from an HDMI source used by something else causes a weird 'popping colors' mode on the actual source, which messes up the image whatever your current settings are and can only be deactivated by doing it again on another source, preferably unused.
Any idea of what causes that?