A shaft of hair is basically colorless.
Cells in the follicle, called melanocytes, add pigment. The pigment,
called melanin, comes in two basic varieties — eumelanin and
phaeomelanin — which combine in different proportions to create the
vast range of hair colors, from jet black to ash blonde. For a long
time researchers assumed that, with age, melanocytes simply
become less efficient at making pigment. That may be partly true. But
recent studies at Harvard University have shown that age brings a
steady decline in the number of these pigment-producing cells.
Not all hairs respond in the same way or at the same time. So the
graying process usually is gradual. You can't prevent graying. Some
people start graying in their 30s, and some not until their 60s.
Genetics likely play a strong role in graying.
People rarely go gray overnight. If they do, it's typically due to
alopecia areata. This condition causes the thicker, darker hairs to
stop growing before it affects the growth of gray hairs — giving the
impression of graying overnight. Alopecia areata eventually causes
roundish patches of hair loss or complete loss of hair on the head or
body. Its cause isn't known..consult dermatologist