So my imagination instantly conjured up a castle as black as night.
Except, of course, that she married into the royal family, which had owned the castle for generations. Her husband's not the typical royal, either. Many noble kings, queens, princes and princess have lived there
But still -- leaving aside that it's probably going to be built from the nearest quarriable rock -- a dark stone is still suitable for a fortress intended to be formidable.
Also, suitable for the heroine's grim childhood.
Black it is. Or possibly a thunder-head gray.