I'm pretty sure the rear camera is color. There is no reason why the front camera couldn't be upgraded to color and the feed split into two. The raw color feed could be used as a dashcam and the 2nd feed could be sent to a B&W filter for AI processing. Then AP2.5 also has an internally facing camera that could be used for additional security (eg. Break-ins).
I'm not saying Tesla would ever do this, but it would be the most efficient way to have dashcam capabilities. Maybe someday someone will figure out how to hack the feed and provide a software based dashcam.