I would go with Field Grey and then what ever colour you seem to think that is in the range of green/grey.
The problem I find with the Osprey titles is that you are at the mercy of the interpretation of the field grey colour by the artist for that title. In the ones I have the colour of field grey is interpretated from a blue/green grey to a more green than grey.
I tend to use the Histoire & Collections 'German Soldiers of World war Two' title, to try and get a better idea of uniform colour. Its full of colour photos of the uniforms from 1939 to 1945. The overcoats illustarted in that book are green/grey. Except the overcoat worn by motorcycle crews which was rubberised and appears to be in a dark brown or faded black colour.
Of couse then its matching the colour from availble ranges or mixing your own.
I'm not sure if overcoats were issued in the 1944 'olive brown' colour which came into wide use in mid 1944. Though it would make sense they were?
I seem to notice that most (or a lot of) painters of WW2 Germans seem to do greatcoats in grey and not field grey? In my opinion great coats seem to have been the same colour of tunics ie field grey and not the colour of the early war trousers ie grey.
I stand to be corrected.