No one has yet mentioned that gold occurs in nature. It is never pure. The most common contaminant is copper. The one bit of authentic Black Hills placer gold I've seen had a distinct red blush caused by high levels of copper. If this alloy was the norm in north-western Europe, calling it 'red gold' seems obvious. Native copper doesn't look anything like gold.

White gold is made with platinum, rhodium, and/or palladium; purple with aluminum; and it is said you can make black gold with antimony.