I don't claim to by a Britannia expert and sadly have no operational; experience of the type but have the following observations to offer which whilst they do not answer the OP's question may add to the intrigue.
It appears that The Original Britannia 102 and the latter 312 and RAF aircraft had flat-topped Throttles the handles being covered in a hard coloured bakelite material, Yellow for Engines 1&2 and Green for Engines 3 and 4.Later export models of the Britannia with side facing Engineers station had rounded tops to the controls which were a uniform colour.
I've consulted Two Airlines Brit technical (312/308) Bristol 300 Series crew notes and the RAF Crews notes AP and found no mention as to why the throttles were colour coded.
Could it have been something to do with the installation and wiring process as the early type leavers are handed?