From what I remember from reading somewhere the past, some mis-matched cross-system combinations will produce a B&W picture. Since you mentioned a multi-platform TV is giving you this trouble, and to risk mentioning the obvious, check that the TV is set to the same platform as the tuner (NTSC?). I'm only guessing here, but maybe you're assuming that the TV automatically sets itself to the signal being fed to it, and it might have to be set manually. Just a guess.

Larry