I am in the process of building a model of an Austin K9 Radio Truck (as used by the RAF in the 1950s). I gather that the rear body interior was painted in a light green colour (similar to eau de nil). Does anyone know what colour was used for the cab interior?

I can't speak for RAF issue, but Army vehicles' cab interiors seem to have been Deep Bronze Green in the 50s and 60s, like the example I am modelling.
If you also have the 1/76 example by Matador (ex BW Models), note the following, assuming the RAF K9 is the same basic vehicle:
Interior - all wrong. Google 'Austin K9 manual' - it is available, and has scale drawings of the interior layout for a D11 equipped vehicle. If you use the same source I did (sorry, I don't recall the address) the interior layout drawings scaled out on my printer when printed at 22%.
LHS - needs tools behind the cab; also, the stowage box is wrong. Most vehicles also had a strip of material to cover the join where the external tent was fixed.
Both sides: The rack of mast sections looks terrible, and the wrong size. No louvres. LHS has a rectangular access cover, hinged at the top near where the louvres are cast. The louvres should also be removed from the rear.
Radio Body Roof: needs ventilators and aerial mounts.
Cab Roof: small modifications to the hip ring.

Eau de Nile is the interior colour of the radio body; there are some photographs which Google Images will find which are in colour.

There may be more, but I can't think of it right now.

Edited to correct 'RHS' to 'LHS' with regard to the access hatch.

I found this lurking in my Favourites - I forgot that I put it there!

http://www.vmarsmanuals.co.uk/archive/files_index.htm

Enjoy!