Never really wondered about it until I got a question about it.
But, if you use the standard clock widget with weather in Sense 6, you often see it as white but then at night it turns black.
So, white during the day and then black when it’s dark (or darkish outside). It seems that it adapts to the local sunrise/sunset times as well, since it’s mostly just white on my Swedish One M7 right now.
And no, there’s no setting for it.