I recall watching some of that (coverage of the Kennedy assassination) and there is even some colour videotape footage of the news coverage in Dallas. And the story behind that was the local affiliate (believe it was NBC) did that for a few minutes before network HQ ordered them to switch back to black and white as that is how the rest of the outlets were covering it.
It took 20-30 minutes for the cameras to warm up, so that is why there was just audio. Television news was still in its infancy in those days.
But I do remember Cronkite going on and on about how Right-Wingers attacked Adlai Stevenson in Dallas a few weeks before, so he just *knew* it had to be a Right-Winger that killed JFK.
Turned out to be a Commie.