(Updated with AllThingsD claims) More sources are supporting the prospect of a gold-colored iPhone 5S. Several people tell TechCrunch and iMore that a gold option is planned, although Apple is allegedly going for a more subtle "champagne" tone that shifts depending on how light hits it. A point of comparison is thought to be the gold model of the defunct iPod mini (see below).
Gold would reportedly be one of the easier colors to anodize for a new iPhone. At least easier than black, which may explain the "slate" tone used with the iPhone 5. Gold might also increase the appeal of the iPhone in China and India, both important markets for Apple.
Still uncertain is how the new option might fit into Apple's existing color lineup. It's speculated that even if metal is gold-tinted, any plastic may be white. In that scenario, there could be up to three 5S options: black/slate, white/silver, and white/gold. Alternately Apple might choose to ditch the silver color, though that may be unlikely given its popularity. The 5S is expected to be announced next month, alongside an all-plastic 5C lineup.
Update:AllThingsD is corroborating the news, including the "champagne" tone. The site's sources add that it will indeed use a white plastic face, leaving only the metal colored.