Why does VHS video go black and white when you fast forward it?

Question

Don't get me wrong, I'm a digital noughts and ones kinda guy, but I was at my gf's mum's last night and she'd "taped" some programme off the T.V. and as most sane people do, fast forwarded through the adverts. It's been so long since I'd seen a video in action and I forgot that it went black and white when you fast forward. But why? Why isn't it in colour?

Answer 1

The film is going too fast when you fast forward.

Answer 2

Not all VHS machines go monochrome when you fast forward. This is a characteristic of your machine. Some very expensive ones keep some sort of colour.

When you fast forward the tape is pulled past the head drum too fast for the heads to properly scan it. That's why the picture breaks up.

Because of the way that the (analogue) signal is recorded onto the tape this imperfect scanning will miss significant parts and the effective frequency received by the head will be much higher too. The machine's electronics can't cope with that.

So, to make sure that you have some sort of picture to help you navigate when fast winding, instead of a screen looking like someone vomited on it, the colour signals are muted.