the way we're using double quotes here, it can indicate either phrase matching, or just that you're quoting something... to avoid confusion, suggest using a single quote when quoting, and reserve the double quote for indicating phrase match.
if you added 'white' as a broad neg, you added -white
and yes, as others said, with that neg, you'll block all of these positive keywords: [white cotton dress] "white cotton dress" white cotton dress
on the broad 'white cotton dress', you won't block everything. for example, synonyms to white, may come thru, like: ivory cotton dresses bleached cotton sundress cotton dresses that match magnolia flowers
you can address those by using modified broad, like: +white +cotton +dress