Question for Lizzie: Was not Brunswick Green used by one (or more, possibly) of the 'big' railway companies?
Green was considered the general colour for passenger engines, black for freight, although there were exceptions with various liveries of lined or unlined schemes, especially with the Great Western Railway (1834-1948) and Southern Railway (1923-1948). The Great Western used an assortment of shades of green, being described as "light" or "dark", depending on the class of loco and how the Chief Mechanical Engineer felt his engine should be presented. It should also be noted that the GWR was the only railway company to maintain it's unique identity from the time of original conception in 1834 to Nationalisation on 1st January 1948. It remained all the way through the "GWR", whereas all other railway companies, of which there was a total of 123, were absorbed into the 'Big Four Grouping' on 1st January 1923. Thus the GWR had it's own liveries from the start to the end, whereas the other three companies after 1923, the London and North Eastern Railway, the London, Midland and Scottish Railway, and Southern Railway inherited numerous liveries from the smaller companies they absorbed. So any comment about livery must start on 1st January 1923, unless you want me to write a book here!
The Southern after 1923 was very much the same as the GWR, adopting it's own colour scheme - livery - for it's engines. Green was certainly typical, but not exclusive, to passenger engines, with black for freight engines. "Olive" and Bulled greens became popular on many engines, along with various shades of light and dark, lined and unlined. Malachite green also became the colour for especially Bulled's West Country and Merchant Navy class's of engines, and some of the earlier built engines were treated with the same colour during the 1940's.
The LMS used particularly Crimson Lake for passenger engines, with black for freight engines in general up to 1948. Sir William A. Stanier, the CME for the LMS, when he introduced his Coronation 4-6-2 Class of loco in 1937 in streamlined form painted 5 of them in a blue and silver livery to match the streamlined coaches that the engines pulled. Later 5 more engines of the class were streamlined as well and painted with a maroon and gilt, with another 15 arriving thereafter. Five more streamlined versions of his Coronation Class were painted in war time black, with non-streamlined versions also so treated.
The LNER used various shades of green and black for passenger engines. Sir Nigel Gresley, the CME for the LNER used Garter Blue for his wonderful A4 pacifics in the main, but for his
Silver Jubilee services in 1935 engines such as A4's 2509
Silver Link, 2510
Quicksilver, 2511
Silver King 2512 and
Silver Fox. was in silver and grey to match the streamlined coaches they pulled.
So, in final answer to your question the 'Big Four' companies used various colour schemes, but a Brunswick Green to my knowledge was not used. However after 1st January 1948 British Railways adopted all that went before, and during an experimental period using various engines over, what were before, "foreign" tracks, different liveries were tried. These included a light blue, an apple green, a light red and..................... Brunswick green. The latter was then adopted for use on passenger engines across the new BR, especially on the Southern Region, were Bulled's engines were rebuilt and had that colour applied. Livery of individual engines was always vital to how an engine looked as the CME's well knew, and by this experimentation by BR it was established which classes of engines should have a particular livery in their particular new Region. Brunswick green was not only used on ex-Bulled engines, but on all passenger engines of the new Standard Classes if designated for passenger service as it was found to not only suit the engines but be hard wearing and capable of hiding so much dirt, as light blue never would! Some of the ex-LMS, ex-LNER, and ex-GWR engines also received this colour. One exception on the passenger engine rule for Brunswick green was in regards to the last steam engine built for BR in 1960; a freight engine, Standard Class 9F 92220
Evening Star which was adorned with Brunswick green livery and a brass plaque.
Finally this is one of my favourite engines, in Brunswick Green, West Country Class 4-6-2 34016
Bodmin