I understand the question but do not have a definitive answer. I think though, that I can offer a hypothesis. The copper is covered by a thin deposit of zinc which has a silver appearance. Upon heating, it is possible that there is some form of physical (or chemical) interaction between the zinc atoms and the copper atoms (just below the zinc coating) such that when light is reflected from it, it has a gold colour, i.e. the crystalline or metallic structure created produces this pecular quality.