Lights on prewar British tanks

[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.

Richard