Just an enthusiast, no real answers, but here's a guess:

These are hazard cameras, designed to be inputs into the guidance algorithms on board. It might make sense for such a camera to be B/W to reduce on board processing required. There's also a glass cover on them, and a lot of dust from the landing, so that may be obscuring true color if the cameras do in fact take color images.

Also they may have just transmitted a lower quality B/W image to get something back to Earth quickly, since higher res images take longer to uplink.