BR blue and grey began being applied from 1965 onwards, after the re-branding of 'British Railways' into 'British Rail'. Chocolate and cream carriages were initially only permitted by the British Rail Board to run on named trains (from 1956 onwards), thus the Western Region invented as many named trains as possible to return to its beloved colour. In 1955, the stock would have been blood and custard / crimson and cream, which was applied to all regions from 1948 onwards, but this proved to weather badly, thus maroon was pioneered on virtually all regions from 1956, except the Southern, where the familiar green livery was permitted. However, I believe that there is a variation in shade between the maroons of the Midland, Eastern and Western Regions.
By the time blue and grey Mk 1s began arriving in force, Western Region steam was in its death throes, with diesel hydraulics having taken over most of the work. The last steam-hauled train out of Paddington was on
11th June 1965, whilst the last Western Region steam locomotives were withdrawn from Tyseley in November 1966. A good mixture of blue and grey stock with regional colours could usually be seen on the Southern Region main line to Bournemouth, where whole coach rakes of the British Rail 'modern image' were fronted by a steam locomotive, right up until July 1967.
This is how I understand proceedings, although I am sure I will be corrected for any erroneous points.