I think I remember seeing something on TV about this a few years ago. It mentioned a technique of quickly flashing black and white, giving the impression of colour, and I think they said Kellogs used it in their cornflake adverts. The picture was all in black and white (as everything was then) but when the housewife poured the cornflakes out of the box, they seemd to ripple with colour. Other products started using the same technique at about the same time.