Congratulations to Buffy_newbie2020 who found that fic looking into another thread
I thought it'd be nice to mention it here too, 'cause the clues in both threads are very different.
So, it's Blue
Summary: Buffy followed her college sweetheart to New York only to find out that he wasn't as loyal as she thought. Instead of crawling back home to mommy and daddy, she decides to strike out on her own in the big city. While meeting new people and enjoying her independence, she learns the true meaning of that timeless idiom, you should never judge a book by its cover.