I just had the same problem on my B17 special. I hear all those comments about the patina, but I chose a black saddle with copper rivets because I loved the contrast.

Luckily I remembered an old tip from QI - HP (brown) Sauce. Just tried it and it works. I used a tiny amount to avoid getting any on the leather which I've just proofided, the couple of rivets I just did look as fresh as new! Sorry if this is a Brit recommending a Brit condiment as advised on a Brit TV show - but they're Brit saddles after all!

Good luck with it.