According to the Oxford English Dictionary, "red-gold" is a "poetic" term meaning "Made of pure or reddish gold."

It dates back to the Old English period.

As specifically gold alloyed with copper, the phrase seems to date only from about 1700, though the editors confess that "In early use [it is] not always easy to distinguish from the simple use of the adjective to describe gold."

So in the ballads it essentially means "gold" regardless.