Thank you all for your very informative replies!
Your response clears up quite a few issues.
I have come across Bart Vanderveen's name several times in my research. I wonder what has become of his files...
I do not know much about the colours. I have come across one postcard image of armoured cars in Greece that is colourized.
Of course these colourized postcards were made from black & white photos to which colour was added at a later date, by a different person, in a different country - so they are by no means the final word on the correct colour.
I have put together a list of named Rolls Royce Armoured cars and several have names relating to families of colours. These names are not exactly a specific paint mix, but for what they are worth, here they are:
There is also T.E. Lawrence's Rolls Royce tender named "Blue Mist". These names seem to indicate greys and sand colours were in use in the Middle East and India from, say, late 1915 on.
They are very rare, but there are colour photos that were taken during WWI:
In looking at another forum (the Landships forum), I found the following discussion related to artillery and transport vehicles which may or may not apply:
The paint mixes that I've been looking at are given in the Handbook for Military Artificers, 1910 and are the same in the 1915 issue which differs in some other respects.
The colour for artillery carriages, engineer transport and other transport vehicles is described as Lead Colour and is a straightforward mixture of white (white lead) and black (lamp black). The instructions say that a lighter colour can be mixed by reducing the amount of black used. In the notes for this colour in the 1915 edition, it is said that this colour is not used on SA Ammunition carts and wagons which are painted in Service Colour. The 1910 edition says that they are painted brown. The notes for Service Colour in both editions say that it is used only on small arm ammunition carts and wagons. This might suggest that Service Colour was a shade of brown but might equally mean that the colour used on these vehicles had changed. The standard of proof reading in both editions is poor and odd details have not been corrected so either interpretation might be valid.
The complete mix for Service colour is this.
Ground white lead 38lbs
Stone ochre 26lbs 8ozs
Burnt Turkey Umber 9lbs 8ozs
Ground lamp black 12ozs
Prussian blue 12ozs
Patent driers 9lbs 8ozs
Raw linseed oil 2 galls 1 quart
Turpentine 3.5 quarts (ie 7 pints)
This mix produces 1cwt of colour.
A second mix is given for Waterproof Service Colour for Canvas Covers. This is the same as the mix above except that the proportions have changed slightly and the raw linseed oil and turpentine are replaced by boiled linseed oil, yellow soap and water.
In all, there are 16 mixes covering 8 colours.
The handbook gives mixes for eight colours and does not always specify their use. Some of the paint mixes are highly specialised but others might be used for almost anything. The colours are Lead (grey), Stone, Service Colour, Red, Black, White, Chocolate and Green. Of these, only Stone seems definitely to be intended for concealment. The red pigment used is Venetian Red which is a brownish colour akin to rust red. The green pigment is Brunswick Green which is rather too blue in hue to be a camouflage colour. Chocolate might be a camouflage colour but its use is not given.
There are two mixes for Stone Colour. The first is this.
Ground white lead 4lbs
Stone ochre ½oz
Turkey umber 1oz
Patent driers 6¼oz
Raw linseed oil for mixing 1lb 6ozs.
Notes accompanying this mix say,"The colour may be varied by the quantity of Turkey umber employed. A common stone colour can be made without adding ochre." This suggests that the colour is not intended as a fixed standard colour and this impression is strengthened by the notes for the second mix.
The second Stone Colour mix is intended for use on the shields and barrel casings of Maxim guns. The mix is this.
Ground white lead 1lb 2ozs
Burnt umber 1.33ozs
Spruce ochre 6ozs
Patent driers 2.67ozs
Varnish, gold size 1/8th pint
Boiled linseed oil 1/6th pint
Turpentine 1/6th pint.
The notes say that this will be enough to apply one coat to the barrel casings of eight Maxim guns. One pound of this mix will coat the front surface of the shields of eight guns. The notes also say,"The colour of the paint may be varied to suit local conditions, such as background &c." This strongly suggests that it is regarded as a camouflage colour. The high proportion of white lead would make this a very pale colour and it would be reasonable to think of it as a desert colour.
The notes refer to varying the shade of the Lead, Stone, Chocolate and Green paints but don't say what the Chocolate and Green were to be used for.
There is nothing in the instructions to indicate whether the paints were matt or gloss in finish and there is no reference to varnishing them although mixes for clear varnishes are given. Camouflage and concealment are not specifically mentioned. The 1915 edition says that vehicle markings are to be applied in white; the 1910 edition doesn't specify a colour.
I understand that the Australian War Memorial Museum Library has a copy of this book. Copies also appear for sale online from time to time. I suggest a Google search. - Google Books has scanned the 1915 edition, but apparently due to Great Britain's stifling copyright restrictions with regards to very old books, you can only see useless snippets of the text online.
Then there is this:
BT Whites British Tank Markings and Names
"Armoured cars used by the Royal Naval Air Service in 1914-15 were generally finished in a light or medium shade of naval grey. An exception was the Royal Marine Artillery Anti-Aircraft Brigades Pierce-Arrow armoured cars (with 2pdr. pom-pom AA guns) supplied in 1915; these were painted Daimler khaki-green, in accordance with Admiralty specifications."
So it appears that the Rolls Royce armoured cars, which started out in the Royal Navy, were sensibly enough originally painted according to naval specifications. After they transferred to the Army, they were presumably painted to Army specifications.
Sorry I couldn't be more definitive. I am not knowledgeable on this topic, I am just passing on what I have read on the Landships forum. I hope this helps