Thank you chewitt for taking time to respond.
I know, buying new TV with HDR support will solved my issues, this is on schedule but for the end of the year :p
I understand the issue, I was not sure if this feature wasn't supported at all on LibreElec, or if that was available for some board. I'm gonna look for temporary solution in the meantime.
Thank you all for your work !
EDIT : this post is solved for me, I have informations that I was looking for but can't find button to mark this as "resolved"