I believe that is in the current iOS Beta, which would possibly mean you'd need the current WatchOS Beta to support it.Don't remember seeing this full screen solid color watch face when wOS 5 was released. Also I can't get it to show. When I add it I only get the 'older' Color watch face. Can others get the solid color watch face to work?
That's just showing you are not running WacthOS Beta, which is only available to developers. It doesn't show you are not running iOS Beta.