Night vision is grayscale.
Mm hmm. mammalian vision is handled through 4 channels not 3 - in addition to cones which can be thought of as an RGB component in a monitor, they also have an intensity sensitive receptor (rods) which only handles intensity, not colour information
Under night-time conditions the intensity is too low to trigger the colour receptors, but the intensity receptors can still fire - more or less. So you're only seeing in a single channel (ie. black and white... or at least monotone).
Technical stuff aside though, I hope to hell they do a better job of the nighteye shader this time.