It is also important to consider Faith's context -- the wearing of pink ribbons by a Puritan woman (who dressed very conservatively and avoided bright color) may be seen as an expression of joyful innocence or the budding sexuality of a young wife (remember, she wants him to sleep beside her, and she can barely restrain herself from kissing him in the street), but it may also be seen as an indication that she is not a "good" Puritan, and thus serve to foreshadow her later presence at the Black Mass.
Brown believes that because Faith is elected to Heaven, he will be as well -- and when he sees her ribbons in the forest, that belief crumbles.
The pink ribbon belonging to Young Goodman Brown's wife, Faith, is an outward symbol of her presence at the meeting in the woods. The last time he saw her hair ribbons, she was sweetly kissing him goodbye before he left for his journey into the forest. He knew that he was off to meet with the devil, and that he was embarking on an evening of evil. Goodman Brown kept telling himself that after this one evening he was going to live a life of goodness with Faith. When he sees her at the meeting, he believes that his Faith is lost (both his wife is lost to evil and his personal faith in goodness has been destroyed). The ribbon floating in the air is a physical reminder that she was there at the devil's meeting in the woods. He is reminded of her presence at the meeting when he returns home to find her wearing the pink ribbons.
I think her pink ribbon functions as a purity symbol