I have been wondering this since the first day, and frankly (as many things with this OS. Sorry. I am a moaner. I knooow! lol) I find it a bit annoying. Leanback doesn't have a way to get into picture settings (any settings from the Action Menu) and neither it has its own picture settings.
What it seems to do is to inherit the picture mode from the previous input channel/app open. Thus changing continually depending on what we used before (which at the beginning even made me thinking something was wrong with my display!).
That gets quite evident (and ugly) when going into home after watching a movie with Expert 1 colour temperature. It also gets paired with the fact that it is 1080p only. I know it isn't the end of the world, but Sony in the PS4 Pro raised the Home resolution to 4K, so I believe it does matter to someone in Sony as well.
Wouldn't it be possible to have, at least, Leanback always using the Android App picture settings, so at least it would stay consistent? Or since Leanback is from Google that's the way it's going to stay until Google will finally update it (hopefully not eclusively for Android O)?
But the picture settings should be appropriate for the homescreen !