Referees used to ‘book’ players who transgressed the rules by writing their name in a small, black book (so they wouldn’t forget) and if that player transgressed again, he would be sent off the field of play, reducing the team’s number of players by one for the remainder of the match.
After the 1966 World Cup when a few players were booked who later professed ignorance of the fact, and were subsequently sent off, a clearer system was needed.
While driving home from a referees’ meeting, the chief referee of the 1966 World Cup, Ken Aston, had a brainwave when he was waiting at a set of traffic lights. Yellow, or amber, is the halfway signal, warning the driver to slow and stop, and red is the stop signal. These highly visible colours would communicate to players and spectators alike exactly what was happening.
And so the red and yellow cards were born, lifted from traffic signals.