Looks like it could be dark sea grey or extra dark sea grey as used on Fleet Air Arm aircraft. The only way to go I guess is to try samples of different greys on scrap plastic card (primed with the primer you're going to use on the model) and see which one looks about right. Remember though that photos are sometimes misleading to how a colour should look in reality due to reproduction techniques and computerised enhancements. Its really a case of which looks right to you. Definitely give the dark sea grey a try though.
I'm having similar issues with my BPBCo 71ft 6 MGB which can have three colours...grey, white on the hull superstructure and dark grey on the decks etc, some say the grey is 'warship grey', some say blue green but in fact it actually looks like duck egg blue, the white should be off white and the dark grey is a dark RAF blue similar to extra dark sea grey (Tamiya neutral grey is close)...........but problem is there is very few if any colour pics of chosen subject...........good thing is though that no one can argue the point without comparison.
Anyway, good luck with your quest and I hope I may have been of some help.
Dave