According to Wikipedia, London Buses route 22 was introduced on May 17, 1909. By 1911 it had evolved into the route that serve as the link between my current home near Putney Commons and Piccadilly Circus. (It was extended from Putney Bridge to Putney Commons in 1916). The route has evolved some since that time mainly being split into two pieces, the northern branch "shortworkings" designeated 22A, 22B, and 22C and later 242. The 22 was later stopped at Piccadilly and the Northern shortworkings were fully separate routes.
Why is this of interest?
Not only is continuous service provided on the same route, a continuously numbered bus route has managed to last nearly 100 years on largely the same route, longer than most rail services.
One could attribute this to bureaucratic intertia, but it also helps locals at least retain knowledge about their transport geography.
While consistent bus numbering was a positive aspect to come out of the reorganization of the London buses through the twentieth century, much was lost in term of information by the use of red for all city buses.
The distinction between the red city buses and green country buses is well known (the green buses have lost their distinction with privatization). However, prior to that, buses along certain services in fact had their own colors (based on which Association was operating the service (Reed p. 10). The "General" was red, and since they were the survivors of consolidation, buses in London are today red. This is clearly much less useful for navigation than one color for each route, but if the companies each operated only one route (or several very similar routes) they would be equivalent.
Having all buses be red might help branding, on the other hand, buses are pretty easy to distinguish based on shape, and don't really need to have a single color for branding at the expense of wasting that parameter for passenger information.
Reference: Reed, J. (2000) London Buses: A Brief History. Capital Transport.