The Samsung Galaxy S6 was launched in 4 different colors (White, Black, Blue and Gold Platinum) and they all will be available in stock by April,10th. Now, Samsung UK's web site lists the Galaxy S6 in five colors, instead of the official four.
The fifth option is "Dark Brown", it also cannot be clicked and disappears if you navigate away from the specific page in the source link. There isn't even an image of the device in Brown.
So, publishing the Brown color option was certainly a mistake. Its existence might be for real though. There's no trace of it on other local Samsung sites, suggesting it may be a UK carrier exclusive. The Galaxy S6 edge does not have a fifth color either, at least not yet.