Color light signals are often installed in a V-configuration.  This is easy to accommodate due to the modular design of the light units.  Facing the signal, the usual aspect placement is green on the left, yellow on the right and red on the bottom. 

Below.  Back of V-configuration color light signal having a single multi-light unit and two doors.

Above.  Three single light units attached together with two angle brackets and a top cross piece.  I added the threaded rod and nuts to the top cross piece to allow the signal to be secured in part to a ceiling rafter.

Older V-configuration color light signal with unusual back door.  We’ve come a long way since then.