Hello Ray,
Yours is one particularly and deceptively un-simple question. It gives us enthusiasts on this topic much scope to disclose our laboriously acquired knowledge. As to buying books "blind", I agree it is niggling to buy one that turns out a dud, but I fear it is an occupational hazard. I do believe though, that the answers to your questions have all been covered in print and on line, and researching in depth may well give you hours of innocent pleasure for years to come.
Attempting something like a useful answer, though, Hornby's Maunsell (pronounced Mansell) Restriction 4 coaches are rightly considered to be classics for accuracy and detail. Also, the lined olive green livery is spot on for newly painted stock from 1926 (when the low window variant was first made), until 1938, when a brighter unlined colour was introduced for service vehicles. At first, priority was given to corridor stock; non-corridor stock started to receive the brighter colour from 1941. In addition, there were (as said above) interim versions of Maunsell livery without lining, and interim versions of Bulleid livery with Maunsell-style numbers on coaches (i.e. round-topped "3"s.) Many subtle variations in hue were created, and were added to by the crimson and cream era.
Efforts would have been made to keep uniformity within a set, but the make-up of a train would vary. The Ocean Liner trains received preference, otherwise the West-of-England came lower down the list. Perhaps unfair to use the term "kaleidoscope", but until the new BR(S) green was introduced in 1956 it would have been quite usual to see a variety of liveries on any journey.
Precision Paints (no connection) produce pre and post war representations of "Malachite" green. They are good enough for many of us. Hornby have in the past (on their "generic" models) produced a perfectly convincing representation of malachite. I have no idea why Hornby rejected this for their current R4 coaches in supposedly post-war livery. No point in frothing, but very sadly they are not for me.
Bachmann have produced several of the post-nationalisation diagrams, but production standards have moved on, and for many modellers the limitations are too great. But the field for Bulleid pre-nationalisation coaches, in today's standards and in an acceptable green is wide open, and in this race I think the winner will take all.
Hth
PB