Why it happens: The cells at the base of your hair follicle produce melanin, but when they grow old and die, new hair grows without color.
When it happens: Most women see their first grays in their 30s, though it's not uncommon to see them in your 20s.
What you can do: All hair will eventually gray (the "when" is usually genetic), and sadly, there is no way to reverse the process once melanin production has diminished, though some products claim to slow it down. Traditional permanent dyes, which often contain peroxide and ammonia, offer the best coverage but can be damaging.
Product to try: Avon Advanced Techniques Color Protection gray root touch-up ($8 at avon.com) will help as an instant fix between colorings.