My keyword target term is "white cotton dress". But i added "white" as broad negative keyword. So my question is will it affect the phrase and exact matched keywords i have used?
Yes. You are in effect preventing your "white cotton dress" keywords from triggering your ads. Negative means "don't show my ads if this keyword is part of the search query".
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
but...
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