The other question is “why don’t you see that noise when the TV is tuned in?” The TV has automatic gain control. When the signal is weak, it will amplify it up to the right level. If the signal is just background noise, then it will amplify that background noise until it’s at the “right level” for a proper TV signal. So, the gain is lower for stronger signals, and very high when there is no signal at all.
the artefacts created in the black and white picture by the colour signal are hardly noticeable, but they are enough to recover the colour from a black and white recording!