I'm just going on recollections of SR stock in the West Country from about 1960 onwards, most of which apart from the Exmouth line was corridor of varying vintages back to Maunsell's time, and seemed to have become green very quickly. Certainly by the time west of Salisbury was transferred to the WR at the end of 1962 it was all green again, with the notable exception of bogie parcels vans, which seem to have stuck with the red livery.
The GWR diesel railcars were about 50-50 to the end between red/cream and green, the latter seeming to be only the more repaints. They were consolidated in the end at two bases, Southall and Worcester, and accounts from both show this mixture. Quite how they qualified initially for the express stock when they were fully in branch line use is not apparent.
The last GWR-pattern passenger stock built, a series of auto-trailers for 14xx push-pull locos in the 1950s, seem to have had every livery imaginable - chocolate/cream, red/cream, lined maroon, plain crimson, so there were all sorts of odd combinations.