Why is TV static noise always black and white?
The color burst is also an indicator that there is a color signal.

This is for compatibility with black and white signals. No color burst means B&amp;W signal, so only decode the luminance signal (no croma).

No signal, no color burst, so the decoder falls back to B&amp;W mode.

Same idea goes to FM stereo/mono. If there is no 19 kHz subcarrier present, then the FM demodulator falls back to mono.