That box is painted Deep Bronze Green as it should be for the time. The modern colour is BS381C Colour Number 224, but this colour was in use and defined as a colour, as E112 in 1912, by the Post Office. Although their post boxes had been painted Deep Bronze Green for the many years prior to this, it was not a defined colour, until they wanted to use it on a postage stamp in 1912.
Bronzing had been used as a technique for centuries. It entailed painting something green, notably cast iron statues, railings etc. then applying bronze dust. The copper in the dust also turned green and we owe the preservation of many cast iron items, open to the elements, to this form of preservation.
British Standards began in about 1902, but obviously they came on stream in order of importance at the time.