I also suspect it would be a type of surface reaction due to the metal composition. "Lead Rot" is usually a dull greyish, powdery affair, and AFAIK pretty much a thing of the past. A yellowish tinge might be due to a higher copper content in the alloy, if I am not mistaken. Push comes to shove, you might enquire with Warlord Games via e-mail.
As a final note, I recall a similar "golden tinge" from Games Workshop miniatures made in the US during the 1990s, where they had to eliminate the lead content earlier than they did in Europe (and the UK). Never had a problem with those, but it's only been close to twenty years that I had them, so I don't know if the evil side effects come in after that.
Generally, I think that if you clean it off properly and quickly add an airtight seal on the figure (e.g. a full basecoat), the problem should be alleviated, if not eliminated.