Pink is a color generally associated with innocence. In YGB, it symbolizes the innocence of Faith who wears it at the beginning of the story as well as Brown's own innocent and immature view of his fellow townspeople. During Brown's real or imagined trip into the woods, when he is at the Black Sabbath, Faith's pink ribbon floats to the ground, and Brown remarks that he has lost his Faith. Of course, the are two Faiths involved here: Brown has lost his innocent wife, and he has also lost is faith (religious and in mankind).
You know the sorry outcome of his disillusionment.