The main example (though Volsunga Saga and 'several old Scottish ballads' were also mentioned) was a translation into English of the Nibelungenlied (we are not told which translation; there are many), using an English-language poetic commonplace which, like 'milk-white', 'nut-brown' and so on, is unexceptional to anyone moderately familiar with the ballad form.

It really is very simple and very prosaic, which is probably why people are still arguing the toss about a question that was answered in broad within an hour of being asked. Some always want complexity, even if they have to invent it themselves.

'Red gold' is a ballad commonplace ('poetic epithet' if you prefer). There is a good reason for that. Pure gold is too soft to make anything functional with. Jewellery, coinage and so on must all be made with a gold alloy if they are to last. Copper has been used for that purpose for a very, very long time indeed, and it gives a reddish colouration to the alloy. That isn't in question; it is a simple matter of fact. That is why 'red gold' was the standard term in ballads, real or pastiche, when referring to gold artefacts and, by extension, other things of value associated with gold. Everybody knew exactly what it meant.

When the publication of Percy's Reliques brought the ballad form back into fashion, the term was naturally taken up by the Romantic Poets and everybody to whom romantic archaism appealed, the Pre-Raphaelites not least among them (for them, red hair became quite an obsession). Naturally, the stock language of the ballad was also used in translations of epic poetry, though as 'michaelr' has pointed out the same terms were used in other languages.

A brief search via Google provides specifics about the metallic content of gold alloys, whether red, rose, yellow, purple, white, green, grey or blue. Most seem reliable enough, as the matter is not a question of speculation or interpretation but of hard fact.