I bought a rack second-hand and it appears to have a layer of semi-metalic paint over the surface of the main cast aluminium piece. I assumed that that's the finish Brompton apply to the rack. . . it must be as the stays have a different finish and they're not detachable or possible to mask cleanly.
The metallic paint appears to have a grey primer underneath so it doesn't wear straight to aluminium. The result is kinda odd, an anodized finish would look much nicer. Perhaps Brompton have switched to an anodized finish in recent years as they have with the brakes.