OK, I think I answered my own question a bit. Doing a little research it appears the conchos we all use are primarily made from cast Zinc alloys, also often called 'pot metal'. These are used because they are easy to work with and have low melting points, but they can really be any variety of metals alloyed with Zinc, with a common one being copper. However, Zinc alloys are apparently pretty good an being corrosion resistant to salts but are not good choices when dealing with acidic foods so they are not good for applications that involve cooking or food service unless they have a coating.
So, I guess I'm still wondering why we don't see conchos discoloring because of long term contact with veg tan leather. Maybe just having copper in the alloy mix isn't enough to cause the problem if the alloy's chemical properties are good (think the difference between steel and stainless steel). So maybe this means German Silver (aka Nickel Silver) is OK too?
Hopefully there are some actual metallurgists or chemists out there that can give a definitive answer?