With my 28 year old set, when I connected the CECB (RF connection only), I got b/w. I still get that or bad color occasionally. Flipping channels or turning the tv off and on again (leaving CECB on) is such an easy fix, I haven't checked into what's doing it.
You know it's not the box, but you can test to see if it's your tv, antenna, the contact, or cable itself. I took my indoor antenna with a recently purchased cable to a friend's to compare antennas. [Mine worked better than the rooftop antenna.] Back at home, I reconnected the cable in the reverse direction it had been originally, and got a better signal. Either I got better contact the second time (I'd been careful the first time) or I relieved a crimp somewhere along the line. But I've swapped CECBs around enough to know that contact with CECB or tv can look fully seated, when it's not.