They rarely have in the past. Even in the BR era, multiple units were green, corridor coaching stock was "blood and custard" (or regional variations) and non-corridor stock maroon. Later all coaching stock was maroon, and later still we went to allover blue for most MUs and blue/grey for LHCS, before everything went blue/grey. Then we got into sectorisation with Swallow, Toothpaste and the rest.
(and there were exceptions - the GWR railcars were maroon at one time, as were the AM9s, and the blue to blue/grey transition went in stages - the Transpennine, Inter City and Cross Country DMUs, and SR express EMUs (CEP, CIG, VEP, REP) went blue/grey abiuyt ten years before otherb multiple units.)