In case you guys are wondering, the description is of a dress used in one of my books. (Miros has probably caught me out.)
And the Regellian culture uses white for "mourning and mortuary personnel", so white during a wedding (or betrothal, as in the case of the full scene) would be a huge faux pas.
Also: The dress is actually something referred to (eventually) as a "forever suit". The untuned version of this basically looks like an off-white graduation or choral robe. On beginning to be tuned, it goes skin-tight, and usually turns a dark color. The item is made of a cloth that is living, and semi-symbiotic, too. It's been rather a good bit of fun writing about this fabric and the length of service it provides. (the cost is pretty extreme, since only a very few even appear in my books, and in one reference in a book I've yet to actually assemble, it's noted that a single suit of this material costs "as much as two large runabouts". But hey, if it's the last piece of formal clothing you'll ever have to buy, perhaps it's worth the expense. (I've not seen a forever suit used as a working uniform yet, just various sorts of formalwear.)