Vickers light tank rear lamps

[George Moore]

An interesting point has come up.
I recently purchased a copy of "Handbook for Light Tanks MkS II - VI).

It is basically descriptions of the design / construction and motive power for the various Mks.

However, in the wiring diagrams and in the "electrical" sections, the rear lamps of the MkIV - V - VI were red and blue ?

Can anyone throw any "light" [ha ha] why one of the lamps would be blue ?

Was it for convoy station keeping at night, or some other purpose ?

Just a query.

George.