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?

Done a bit of digging. . . . . . . . . . . .Well, I asked my chum David F in Bovvy.

It is the early version of a convoy light, tanks not having an external, rear mounted and underslung diff casing to conveniently paint white; low vis and not visible from above.

R.