Pure white is worn by the morally pure.
In western cultures, white is used to symbolize moral purity. This woman wears a dress that's as clean as she is. This is traditional for a bride on her wedding day, debutante at her coming out party, and Virgin Sacrifices. It might be relevant today because of the characters being religious or bringing back Old-School Chivalry. If there are moral expectations you can expect some Slut Shaming by the Moral Guardians for those who don't remain "pure". An Ethical Slut doesn't take the idea to seriously though. In the old times, any unmarried women was expected to remain chaste so she could be identified by the color she was wearing. The Ingenue might wear white just for the virginal symbolism. A Fairytale Wedding Dress will be pure white unless it has a little bit of pink or something as girly on it. The girlyness is probably done away with for the virgin's dress though because she's so mature all she needs is a dress that's simple, understated and still strikingly beautiful and white. Technically she's a Woman in White, but she doesn't necessarily have the importance and style that that character has, as the color is expected of the bride, which takes the mystery out of her. Examples can range from the saint like, to a sexually inexperienced woman who wanted to "lose it" or an experienced one who "renewed it" or any mention of sexuality white wearing white. This is the basis of the Blood-Splattered Wedding Dress. Compare Gold and White Are Divine and True Blue Femininity.