If we were really there, the comet would look black, black, black and more black. And the shadows would be slightly, but probably imperceptibly, darker. The images are extremely enhanced to exagerrate the tiny variations of deep black.
The question of colour is interesting. Space probe cameras don't have RGB sensors, they're monochrome with lots and lots of switchable filters for specific purposes, like seeing seeing specific gases like nitrogen monohydride or a mineral like orthopyroxene, and many are in UV or IR. It's a bonus if the science lets you make more or less true colour images too.
The resolution will be much better when the probe gets close to the nucleus as the narrow field camera won't be able to see all of it at once. What I've seen so far seems to be only part of the full frame.