It's purely a decorative effect. There are at least two companies offering brass plating kits, but they cost more than I want to spend and it also looks as if you have to put down a layer of copper before attempting the brass. Anodising did cross my mind.
Think I'll buy a small tin of Humbrol brass coloured paint and see what it looks like on a piece of scrap. May not look realistic but might still look acceptable (even a regular solid colour could look better than diecast aluminium!) - especially for a cost of ~£2
.