[Leica] Complimentary flash filter

[George Huczek <ghuczek@sk.sympatico.ca>]

Recently I read of a photographer who uses a colour gel on his flash, while
using a complimentary colour filter on the camera lens.  I think he had a
green filter on the lens, and a magenta gel on the flash, or vice versa.
Can someone please tell me what practical differences these filtering
techniques have?  I understand the theory of why this works, but I am
wondering what differences would actually appear on the photos if one did
this.  He was using a standard focal length lens on a N***n to photograph
people, using TriX film.