I find the plum colour of some of the the trigger blocks on Webley Service rifles to be quite an attractive feature, along with the similar colour on some of the Webley Junior trigger block/strap castings.
I was told many years ago by a Webley contact that this colour was the result of different castings from various manufacturers reacting in different ways with the blueing salts, which is why some are 'Plum' and some are not.