Dear Nigel
If you look at the recipe for the stain, it would best be described as weathered creosote. It was not a pure black so could be described as a grey as that is not black.
Look at the colour of the wooden pilons etc
It is as ever the problem with a verbal description and attempting to paint this upon a canvas or paper gives you the problems. Start with oak and a heavy black wash. This would likely get close to the colour.
Gohlke (1914 rp 1977) Geschichte der gasamten Feuerwaffen bis 1850, p187 states:
"Sachsen
Geschutzanstrich im Siebenjahrigen Kriege mit Messingbeschlagen (Landesfarbe!). In den napoleonischen Kriegen und auch spaterhin [z.B.Heine (1834)] dunkelgrau mit Messingbeschlagen. Munitions- und Sanitatswagen: schwarz mit roten Decken. Grauer Geschutzantrich auch noch 1866, 1870-71 bis etwa 1872."
That is a view of the German Author c1938 when this was written and added to the Gohthe (1914) when the 1977 edition was compiled.
There is a fine Saxon 24-pdr outside Firepower in Woolwich, other carriages in Saxony and Copenhagen. These are all as I have attempted to describe.
Interesting that he used the term Messingbeschlagen [brass fittings]. None of the illustration that I have seen or the extent carriages that I have studied would be what I would describe as grey.
Stephen