nigel
British armour cars sometime have two tones colour in which dark green and black colour are used. Check it out with this picture here:
http://afvinteriors.hobbyvista.com/ferret/ferret3.html
If you want to put more details on your model, you can also find some interior pictures in here:
http://afvinteriors.hobbyvista.com/ferret/ferret1.html
Hope it helps. Looks like you father Ferret served in Middle East or North Africa. Colour might be different there.
BTW, this may be old news, in case some of you don't know, AFV Interiors site is back
. It is now here
http://afvinteriors.hobbyvista.com/