I at least have two different workarounds: turn Reduce Transparency on or manually switch to a different screen saver. I don't want their presence known when I am actively using the device. I think of screen savers as either on or off. I would ask Apple to change the behavior, at least as it pertains to screen savers. I believe the Apple TV is working as designed. My display was 'back to normal' this morning because the screen saver kicked in while I was in bed and happened to be on a different one when I woke up. The reason the issue did not clear up when navigating the menu is because the screen saver pauses in place and continues to show through. Similar results happen with other screen savers. It was on 'Passing over the Longji rice terraces in Guangxi Province, China.' Very bright, green, and beautiful indeed. The reason the dark green background happened is a combination of Reduce Transparency being off and a screen saver with a lot of green. I had not noticed before the ways transparency affects the display. You got me thinking and doing some experimenting. Things were (seemed) back to normal when I woke up this morning, and now I know why. (I thought it might be related to the charge level on the remote since that correlated the first time, but it is at 100% as I write.) Others are experiencing this issue as well. I don't know what caused it to revert back the first time. This is the second time it has happened to me. All other colors look great (app icons, top shelf, video streams). Please note that this is only the system background. (Applying color filters will obviously affect the background as well.) But Reduce Transparency shouldn't affect color. Turning that on makes the system background plain gray. The one setting that did seem to affect the background color was Reduce Transparency. (Excepting that I am already on latest OS.) I even went through the color balance process.

Followed the steps but they not resolve the issue. Setting the mode to light results in a light green background. System background of Apple TV 4K (2021) turned green and will not go back to dark gray (dark mode setting).

Green screen background 4k