Video color looks muddy when in fullscreen in external monitor (TV)

I am not sure if it is a very special bug or a feature: When I use my TV as an external display and bring go full screen with a video the colors look muddy (less saturation). The same video without full screen looks as expected. If I don’t have this issue in Firefox. Any suggestions?

See: https://www.reddit.com/r/mullvadvpn/comments/1amwb7z/mullvad_browser_full_screen_colors_muddy/

If there are no immediate obvious cause, we can close this issue (also tracked on Github).