On its App Store Marketing Guidelines page, Apple has diagrams of its devices that are to be used by app developers to promote their work. Developers can show a screenshot of their app on an Apple iPhone 5s, Apple iPad Air or an Apple iPad mini with Retina display in silver or space gray, or on an Apple iPhone 5c in white or blue.
Why keep the gold Apple iPhone 5s away from developers' marketing? Well, if you've seen Apple's own ads for its new full-featured iPhone, you might notice that the gold version shows up quite often. It could be that Apple wants to keep the gold colored iPhone 5s for its own promotional material. Of the three color options, gold was the most popular out of the gate and those looking for a gold iPhone 5s were met by shortages. Apple has since increased production of the gold model of its new flagship smartphone.
Apple asks developers not to use gold Apple iPhone 5s to promote their apps
source: Apple via MacRumors