Hi upnick
There are many others more expert than I on this forum, but since you haven't had an answer yet, let me try and kick off some help for you.
By the mid sixties MK1 stock would mostly be in either maroon or green with a small number of vehicles in Chocolate/Cream. What happened was this. Most MK1 vehicles built pre 1956 or so were delivered new in Crimson and Cream. After 1956/57 BR granted the regions some automony in what colors to use with the result that the Southern Region reverted to green. The Eastern, and the Midland regions pretty much stuck with Maroon, and the Western region painted some of it's coaching stock back into chocolate and cream. As far as I know they didn't paint all of their coaches - just ones - mostly rakes - used in premier expresses
Thus to answer your question in detail we'd need to know what BR region you are leaning towards. If you favour the Western Region you'd be Ok to run your expresses in mostly chocolate and cream with Maroon for some of the vehicles and for secondary passenger trains. For Eastern and Midland stick with mostly maroon and for the Southern - mostly green
Of course these are not hard and fast rules, Coaches tended to stray so there is modellers license to be had if you wish. Also by the mid 60's the colours used to that point were just starting to give way to blue/gray for corridor coaches with blue for non-corridor
As to what sorts of coaches to mix together to make a set - I'll leave that to the experts except to say that you can't go too far wrong with a Brake 3rd or Brake composite at either end with other vehicles making up a mix of first and second class seats - mostly second of course. Dining cars are a minefield to themselves as some dining vechicles actually have no kitchen and must be paired with a kitchen car. Your safest bet is probably a 'RMB' which is a buffet car
Hope this helps
At least we'll probably get some of the folks far more expert than I providing better details. Perhaps someone can post ISBN numbers of helpful books etc... I know Model Rail Magazine in particular has carried a recent series about train fromations
It sounds from your question as though you're operating 'N' guage - correct..?
Norm