Agreed on Flash.
Diana, however? Her costume looked about that dark in BvS too. By now, the armor has been in use for about 100 years from her own movie. It's possible that it's meant to show that the armor has either been darkened from continuous use or that Diana has neglected its maintenance since she went into at least semi-retirement.
That's just my personal fan-theory anyway. It could also just be Snyder and his irritating love affair with filters, of course.