When someone goes through a very stressful or traumatic experience, sometimes their hair changes color. Documented cases of survivors from car wrecks, personal tragedy, and other losses show people-even young children-coming away from those experiences with a shock of gray hair as a result of the trauma. It is a strange phenomenon that sometimes occurs. It can happen suddenly, or the effects of aging can be expedited more quickly if one is under consistent and prolonged stress. When Faulkner presents Miss Emily as having her hair color change, it is 6 months after Homer Barron disappeared. No one had seen her during those six months, and the first time that they did, she had grown fat and had gray hair. So, that could symbolize the great stress that she went through with the Homer Barron situation; I guess the trauma of him not wanting to be with her, then her unusual and disturbing reaction to that threat of abandonment took its toll on her. It aged her quickly. Faulkner was probably symbolizing that she had experienced a significant change in those 6 months, and that from that point on, her life was forever altered.
The gray hair mention also helps us to fill in the very gruesome missing details of the corpse in the attic, because when they find it, they also find an "iron-gray" hair lying next to it. Faulkner made sure that we knew she had gray hair after Homer's disappearance. He made sure we knew that, so that when the townspeople find a gray hair next to his supposed body, they would understand just who had been lying next to it.
Emily hair turning gray represented the decaying of the old south.