Red gold is an alloy produced by mixing pure gold with copper, and omitting or reducing any silver content.
The expression "red gold" is actually an oxymoron, because gold, at least pure gold, is yellow, meaning there is actually no such thing as red gold.
Red Gold Alloys
An alloy is a homogeneous mixture of two or more metallic elements. Alloys are valuable because they can be engineered to possess different qualities which cannot be created with pure metals. The most common admetal components in gold alloys are copper and silver. Traditionally, most gold alloys were a mixture of all three metals, gold, silver, and copper, often with other trace metals. These mixtures usually produce a pleasant yellow colour similar to pure gold itself. Creating a red gold alloy is quite simple, omit the silver, and use just gold and copper. The result is a red colour, which varies only with the different proportions of the two metals. Some people prefer red gold compared with yellow gold. We don't! It is also possible to make "white gold" alloys, and other colours, by altering the proportions of the constituent components.
Red Gold Coin Alloys
Most gold coins, except for some modern bullion coins, are made of 90% or 91.67% gold. If copper is used as the sole other metal, the coin will appear red; if copper is used with silver, the result will be a pleasant yellow colour. Sometimes silver is used without any copper, and this produces a slightly greenish yellow, often called green gold. As gold coins were almost always produced in high carat alloys, no coins were are made in "white gold" alloys. To create "white" gold alloys, or more accurately grey, the some copper content.
We recently received a complaint from a customer, and an accusation of fraud, because five modern sovereigns we supplied looked red and coppery. They do, because the Royal Mint uses a gold / copper alloy which looks to us rather cheap and nasty, rather than add a small percentage of silver, which would produce a pleasant yellow "gold" colour. We have told numerous Mint personnel about this on various occasions, dating back to about 2000. Of course, being the British Royal Mint, they know best, and refuse to listen to any advice from mere coin dealers, despite the fact that most dealers are in closer contact with collectors (some dealers are also collectors), and often have decades of experience and expertise, whereas many of the Mint officials know very little about coins, but perhaps have a business degree in Marketing or similar.
Rather paradoxically, this customer originally ordered "our choice bullion" half sovereigns, which would possibly have been George V, and contained some silver, so looked a nicer yellow colour, but she wanted coin like her friend had previously bought, so paid extra to "upgrade" her order to modern "newly minted" ones. She would probably not appreciate the humour or irony of the situation, so for a quiet life, we have not yet informed her!
Other Gold Resources
We have a number of other pages which may be of interest to you.
Modern Gold Sovereign Alloy Content (22 Carat Gold)
Bullion Coin Selector Page
Royal Mint 2011 Bi-Metallic Gold Two Pound Coin Specification
18 Carat Gold Alloy Content
9 Carat Gold Alloy Content