Why do you think Faulkner emphasizes the way Miss Emily's hair turned gray- and what do you think is significant about the time it started to happen?

Gray hair, of course, is correlative with age.  Emily's hair plays a symbolic role throughout.  First, in Section II, the anonymous narrator notes:  "She was sick for a long time.  When we saw her again, her hair was cut short, making her look
like a girl, with a vague resemblance to those angels in colored church windows-sort of tragic and serene."  Angels are typically depicted as blonde and youthful-looking.  The new cut and color coincide with Emily's burgeoning romance with Homer. 

However, the next time her hair is mentioned, the narrator observes, "When we next saw Miss Emily, she had grown fat and her hair was turning gray."  The relationship has apparently gone sour with Homer, and her looks reflect her state of mind. Shortly thereafter, the narrator recalls, "Up to the day of her death at seventy-four it was still that vigorous iron-gray, like the hair of an active man."

Think about the symbolic parallels:  gray is the color of metal; metal equates with hardness and strength.  It is also interesting that the similie "like a man" is used, for Emily is not granted much femininity throughout the story.

The final description of her hair is repeated in the last line   On her pillow, "we saw a long strand of iron-gray hair."  Emily has died from a lack of love, from old age, from neglect.  Her hair tells the story throughout.