Andrew. Warship grey deck brown seems to be a bit of a subjective thing, in that if you were to compare 10 pictures of the same ship taken at different times they will all look to be a different grey brown . There may be a "dulux" colour mix spec that RN ships are painted, but this not neccessarily the best colour to do a working boat.
I made a Titanic a while ago and there was considerable opinion "out there" as to what colours were best <for the deck>, and even using different colours at different scales !!
We have had this same question as a post some time ago and I think the outcome was, choose a grey brown you are happy with, easy as that.
Ashley
thats what I say !!!