I suspect that this may have had something to do with distinguishing long-distance or express trains from local or commuters. On the old GER lines, suburban electrics were green, loco-hauled expresses had corporate BR colours (remember 'blood and custard' coaches?), then the later Clacton expresses had the 'jaffa-cake' orange and brown. SWT, before the change to SWR, operated a similar policy; they had basically white long-distance (for them!) expresses, blue for outer suburbans, and red for inner suburban.
Incidentally, those of us on the ex-GER main lines had steam withdrawn for local services in 1949, though we got Britannias for the Norwich expresses before anywhere else. However for a kid growing up in the mid fifties to early sixties, variety was all around. Green 1500v DC electrics (slammers and sliders), Britannias and B1's, occasional J15's, and more, then Type 4 1-Co-Co-1 English Electrics (TOPS 44? not too up on the reclassification), Type 2 Brush A1A-A1A (31's) North British (Class15?) freight diesels, and a couple of Railbuses at Romford - then in the sixties after the 25Kv conversion, the 'jaffa-cake' Clacton electrics. Penalty of that? No models of the electrics or the NBL diesels, and only in the last couple of years a J15.
Why don't i model that era? What with?