Because the frequencies used to provide the color information are
slightly different from the ones that provide the b/w information, the
set may be picking up strong enough B/W signals and too weak color
signals, depending on the channel and the antenna orientation, shape,
room reflections, and phase of the moon.

Many older sets have a "color killer" control that if set marginally
can cause the color to drop out. The control was primarily to allow
killing the color circuits when a B/W signal was being transmitted by
the tv station, to eliminate color sparkling in portions of the B/W
picture. It may be a back panel adjustment if the set has one.

H. R.(Bob) Hofmann