* Gray hair is the absence of pigment: Gray hair isn't a color in itself; it's the result of melanocytes (the cells that produce pigment) slowing down or stopping melanin production in the hair follicle.
* The process: As you age, the melanin production gradually decreases. This can happen at different rates and in different patterns for each person.
* Appearance: For someone with dark brown hair, the hair strands will become gray as they lose pigment. This can give an overall appearance of the hair lightening as the gray strands mix with the remaining dark brown hairs. However, the dark brown hairs themselves aren't necessarily changing color; they are just being diluted by the increasing number of gray hairs.
In summary: The perception of hair lightening is due to the increasing presence of white/gray hairs, not necessarily a change in the existing dark brown hairs.