The first S's were blond, both Mercury and Airsporter, circa 1982?? Very blond. Mid to late 80's a much darker colour done similar to the Century. And later still some even darker S's came out, though few had much stripe to the colour. Some only have chequering on the grip and then the Challenger came out as the flagship break barrel.
I suspect yours is either a lucky late one or a refurbished which has had some staining put in. I'm hoping its the former and then you have a very tasty example well worth cherishing. Collectors of BSA's would love it.

Mint in box then well worth a hefty premium, though unlikely to break that £300 barrier (the blonds $250). Excellent then still a good premium but under £200. Well used but nice then as a shooter and not that much, £140. A good amount were made, but then most were heavily used and abused so good examples are not so common.
The S's were expensive compared to the competition. Getting to the price range of a FWB Sport or HW35, both of which were better mousetraps.