Among the issues that Nexus owners have been reporting lately, is the issue that colors while watching video on their Nexus 5X, 6P, or Player, are muted. Users are saying that the blacks are not true black. This is because (at least for the Nexus Player) an internal setting got switched, changing the RGB values from full RGB display (with values from 0 to 255) to RGB Limited (16-235).
This issue has been reported at places like the Google Help Forum, XDA, and Reddit. There seems to be a similar, yet different issue with the Nexus 5X and 6P. This and the Nexus Player issue have been addressed by a Google employee, as quoted below:
Hey everyone, We understand the severity of this issue. It’s a widespread problem impacting all Nexus Players on 6.0. We’re working as fast as possible to resolve it.
We also noticed similar problems on Nexus 6P and 5X, as reported in #14; however, those had a different root-cause. Those have been fixed internally, and will be included in a future MR.
However, according to Android Police, this issue does not seem to be affecting Marshmallow on all Nexus devices (5, 6). Hopefully this issue gets addressed soon in an upcoming OTA update (maybe along with our new emoji!).
Source: Google Product Forums, Google Issue Tracker, Reddit