What to Wear
For a black tie optional wedding, men can wear a tuxedo, but it's not a requirement. If they end up going tuxedo-less, though, they should wear a formal suit and tie instead.
Women can go the long evening gown route, but a dressy suit/jumpsuit or shorter cocktail dress is fine too.
Dark colors are not required; let your personality shine through! Below are some ideas if you're still unsure.