Color issues with Apple TV 4K on Oppo BDP-105 connected to Panasonic plasma TV

Why are the colors on my TV set, which is hooked to an Apple TV 4k bland, when a movie or TV show is shown in the Apple TV app? NB: The opening screens, even in the Apple TV app look fine, and colors in Netflix or Prime Video are also fine.

Apple TV4k with TV OS 26.0.1 connected by HDMI to a Oppo BDP-105 on the front HDMI-in, then connected to my Panasonic plasma TV.

[Edited by Moderator]