Headset: Rift CV1
System: GeForce GTX 1060 6GB, Intel Core i5-6400, MultiSync PA272W display.
Non-standard Peripherals: None
Here's an unedited PNG screenshot that shows the posterization (or color banding) in dark areas. That's exactly how it looks from both the headset and mirrored desktop window. It's from one of the Australian stages at midnight with clear skies and headlights off. It's noticeable on any stage where the environment isn't strongly illuminated, i.e. under dense folliage when it's overcast, or anywhere between sunset and sunrise.
The colors seem fine when I'm using a TV as my display, but I've only done one quick test. Non-VR mode is also fine.
It's been present since the first VR release of DR2.