Britannia Throttle Levers
 

Let`s guess,red for 1 & 2,green for 3 & 4,same as your socks should be...

You might care to try the Bristol Britannia Preservation Society, although they are preserving one aircraft only.

However, I'm sure that team must have some Britannia experts on board.

It avoids pilots having to write LEFT and RIGHT on the backs of their hands!!:)

CJ, on the gloves ,dear boy.....!..heaven forbid,sweaty hands on throttles.....!!

You never know what you might catch.

It's because co-pilots can't count. "Shut down number three" would create confusion, whereas "shut down the left-hand green one" would be understood.

Sounds logical to me...….

I asked two ex-Britannia captains about multi-coloured throttles this morning and I got two blank stares. "I thought they were all black" was the response.

Perhaps if the OP said which colours we are talking about,it may be helpful...
edit; having just looked at a few cockpit photos,it seems that 1 & 2 were yellow,,3 & 4 were green;others were all yellow ,or a mix of black yellow and green.......!!!!

A book I have (Specialist Aircrew) says that the pilots in the Britannia did not manage the thrust levers, they were handled by the FE, in the RAF anyway

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.
https://cimg3.ibsrv.net/gimg/pprune....a0b2a36cc7.jpg
https://cimg7.ibsrv.net/gimg/pprune....9a8bafcdf8.jpg
Could it have been something to do with the installation and wiring process as the early type leavers are handed?

From memory; RAF Britannias had identical black knobs on all the thrust levers.

Throttle Lever Colours
 

This may not answer the question fully but, as a member of the team preserving XM496 at Kemble, my answer when asked by visitors why the throttles are topped with different colours, is usually that red for port and green for starboard would be logical but red levers and red lights on a flight deck are often for emergency use or are warning indications so the manufacturer presumably chose yellow for numbers 1 and 2 as a safer alternative. I may be wrong, in fact as Britannia aircrew from 50 plus years ago, I can't remember for certain what colours the levers were originally and the manuals are all in black and white!