In those days the colour (or rather the livery) of a town's buses were part of what would today be called its 'corporate identity'. Most places had red or green but Newcastle stood out in having yellow buses. When the buses in London were privatized the streets of the capital became a riot (in the negative sense) of colours before the powers that be wisely realized the damage this was doing to the capital's image and insisted that buses should be red again. Unfortunately this was not the case with Newcastle yellow. However, as the 1948 illustration shows, the yellow livery itself was an innovation. Corporation motor buses were originally blue, while the trolleybuses and trams appear to have been yellow and orange. I like the image of the Corporation single-decker speeding through the Northumbrian countryside.
The Trolleybuses of Newcastle upon Tyne 1935-1966 by T.P.Canneaux and N.H.Hanson Newcastle upon Tyne City Libraries 1985