Lights on prewar British tanks

Originally posted by Richard Farrant
Does anyone know the reason why some prewar British tanks, had two rear lights, one red and the other blue? They were controlled by one switch so only one was lit at one time. It may have been some form of signal or warning, perhaps. It is mentioned in the 1939 edition of Manual of Driving and Maintenance for Mechanical vehicles (Tracked), but not why.

It would be a huge clue if there was a similarly operated green one at the front perhaps, I'm guessing here and do not recall the correct order but don't army convoys carry a green flag at the head and a blue one on the last vehicle? Or is it the other way round?

R.