I tested a bit. For some reason I had to reflash the whole image as the uptate with tar file would give me a black screen.
Once full image in, it worked very well ! 
YUV 4:4:4, 12 bpc for now since I didn't encounter any issue with pass through so far, very happy, Thanks again 
Thank you so much for this sky42 !
I will try it and give a feedback
Interesting but I don't think it would work. I used edid-decode native command to get the edid in human readable form and the only thing I could see is about YUV :
Supports YCbCr 4:4:4
Supports YCbCr 4:2:2
no support Y/N for RGB seems available.
Also, let's say we manage to disable RGB in the edid somehow, looking at the intel driver code above, it would still return RGB which will probably lead to a garbled colors screen.
I had looked at that wiki but didn't found how the kernel is integrated there. So I will take your advice and start to try to build an generic image that works. Will probably start by installing a brand new ubuntu on a different computer and play with that, will take me some time though 
As for the things to be changed, it's right here, post #4, just a few lines in intel_hdmi.c
I tried already RGB full/RGB limited using different options and different source devices. While I can get close the YUV input in terms of quality, it is still a bit less vivid.
So how do I build a hacked linux kernel and use that in the LE build ? Does LE build just takes the kernel of the machine it's compiled on, or how does it work ?
ain't that for RGB limited ?
I need YUV so I guess the only option is to hack the kernel as smp pointed out. Question is : how do I do that ?
In the topic where this was mentioned, it points to a bit of code that I wasn't able to find in the GIT source repository :
Posts by smp - Page 2 - LibreELEC Forum
I looked and I can't find a/drivers/gpu/drm/i915/display/intel_hdmi.c anywhere ? I must be missing something. My Unix dev days are from 20 years ago 
Hi,
I've installed the latest LE 12 https://releases.libreelec.tv/LibreELEC-Gene…-11.95.1.img.gz on my intel core i5 12500T (Intel 12th Gen) with HDMI 2.0/HDR 
output.
Everything is going fine except the color space witch outputs RGB. My projector needs YUV, otherwise it thinks it's a PC displaying Powerpoint slides and switches to low contrast/high luminosity which is very bad for video content.
I've tried searching on this forum how to do it but it's very confusing, not clear if I need to modify the code and recompile, which I don't know how to do or if a simple command at start will do it.
Can anyone help ?
Thanks in advance.
