As far as I can tell, the colour should be; Extra dark sea grey. The same shade as Xtracolor X5 or humbrol No 27. It's a dark blue/grey colour, if that helps.
The underside would probably be RAF sky. (Xtracolor X7/Humbrol No 90)
If you cannot get hold of paint charts for those makers, do you have any old model magazines where models may have been built in those colours?, that may give you an idea of how to procede.
The paint on a WW2 aircraft would probably fade anyway, so a shade or two out may not matter too much.