Thanks Roy for the input.
I've been looking over the parts here of WW1 period vehicles and trying to get a colour match to swatch codes. Wheels have the best paint retention although some chassis do hide bits of paint. Aside from washing the paint areas down some areas have been gently rubbed down with wet and dry sandpaper to revel the progressive layers of paint. The swatches used are BS 381. The grey paint applied first on a 1916 Leyland, appears to be Dark Admiralty Grey. Progressive layers appear as Dark Sea Grey and Medium Sea Grey. These latter two colours continue on into the 1920's being found on latter chassis wheels.
Thornycroft pre WW1 have the colour Medium Sea Grey moving on latter to Dark Sea Grey.
Aircraft grey is also in there as a layer as well as being found as well on other mechanical items of the time.
The variation between the dark shades of grey is small, as is the same with the light shades. This would follow on from Roy's comment to the colour blending being not a specific measurement.
These colours are only the lower layers, the more recent upper paint layers were not matched as they would be from latter commercial use colour schemes.
I don't have a battle ship or the like to compare paint samples to, but considering the names used one does suspect a naval link to the original colour.
It was an interesting afternoon trying to take these samples. A day was chosen without strong direct sunlight, giving a diffuse light without glare, for matching colour. I have also scrapped off paint samples and placed them in envelopes as a future record.
Doug