Gold can indeed be reddish, and the term Red Gold is given it in such cases by goldsmiths and jewellers - but I suspect that the main reason for preferring to use the term "Red Gold" in ballads and epics and such would often have been to link it in the mind with blood.

"Red gold" can push over a switch in the mind somehow which "gold" on its own would not. It raises the stakes. I'm sure that's why Kipling liked to use it, for example in the Rhyme of True Thomas

"And some they give me the good red gold,
And some they give me the white money,
And some they give me a clout o' meal,
For they be people o' low degree."