Thank you for the hard work!
With my Receiver (Denon AVR-X1600H) and TV (Philips 55OLED804/12), I also have the washed-out colour problem:
(RPi4 with LibreELEC) vs
(Builtin video player) and
(Windows 10 with some Kodi Nightly)
Curiously, my receiver says the following about the video signal:
- Resolution: 4K:24Hz
- HDR: HDR10
- Color Space: RGB
- Pixel Depth: 8bits
When with the Windows 10 input it says:
- Resolution: 4K:24Hz
- HDR: HDR10
- Color Space: YCbCr 4:2:2 BT.2020
- Pixel Depth: 12bits
The attached hdr.zip contains edid.dat and debug log. I hope it helps unvover the problem.
Addendum: Just saw your response to noggin and forced 1080p playback; still only 8 bit RGB and washed-out colours.