Why my color TV becomes black and white when reception is not good?

In my colour tv, sometimes when the reception is not good, the colour pictures become black and white. What causes that? Which part of the video signal is missing?

Magnetra:D

If you don't have a colour image the part of the signal that is missing is the crominance.

The video signal was originally desined to black and white whose luminanse signal is build, afterward it was introduced crominance signal. The crominance signal is modulated in a subcarrier inside the video signal, the frequency and composition of this subcarrier varies from technologie to tecnologie ( NTSC, PAL, PAL-M , SECAM) when the signal is weak the subcarrier maynot be detected and so it is not demodulated.