Prototype images of the iPhone 5s surfaced on Twitter today. The photos reveal that there was a Black and Slate color variant, which was most likely done to conceal the product as it underwent testing process.
Historically the iPhone 5 came in two colors- Black and Slate, and White and Silver. The iPhone 5s added three new hues- space grey, gold and silver.
Twitter user @DongleBookPro mentioned how the unreleased color could mean that Apple was considering releasing the iPhone 5s in Black and Slate, or that the color case was used to conceal the device.
The image date shows that the prototype was created on December 2012, 3 months after the iPhone 5 was released and about 9 months before the iPhone 5s was released to the public.
The iPhone 5 in Black and Slate was a popular model. However, Apple replaced it with space gray to represent evolving color options on Apple devices.