If you take a look at promotional materials for various apps these days, you'll notice something very interesting -- none of the photos show an app running on a gold iPhone 5s. MacRumors is reporting that Apple is now asking developers to use only certain colors of devices when producing photos and videos for marketing purposes.
In the App Store Marketing Guidelines, Apple provides a number of product images with blank display areas where developers paste their screenshots for marketing purposes. At this time, the product colors are limited to the iPhone 5s, iPad Air and iPad mini only showing up in silver and space gray; the iPhone 5c in blue and white; and the iPod touch in blue and space gray.
You'll notice that the gold iPhone 5s isn't included in that list, and that's just for static images. If developers wish to use devices in custom photography and video, they need to get approval from Apple, but they're told exactly what they can feature in their imagery -- "Feature only the most current Apple products in the following finishes or colors: iPhone 5s in silver or space gray, iPhone 5c in white or blue, iPad Air in silver or space gray, and iPad mini in silver or space gray."
Eric Slivka at MacRumors posits that "Apple prefers to keep focus on the distinctive color to itself," using the gold model in much of its own TV and print advertising. The ban on developers using the gold iPhone 5s in promotional materials could be a way to keep demand for the model in check until the company is able to boost production.