Why Hair Is Darker When Wet
A: Well, what you are seeing isn’t actually a color change. Gray hair looks “white” because it is lacking in pigment (or has a significantly reduced amount of pigment).
When hair turns gray and loses its natural pigmentation, the hair is even more translucent and reflects and refracts more light. When gray hair is wet, it is weighted down and plastered against the other hairs on the head by moisture, which makes the hair more transparent. The wet hair magnifies the color of any other hairs nearby and isn’t easily penetrated by light, so the hair itself looks darker.
The exact shade of the “apparent” color of the gray hair when wet depends on the pigment in the hair that is not gray. I know many men who are salt and pepper gray, whose hair appears inky black when wet.
For what it’s worth, this is the exact same principle that causes any hair color to appear darker when the hair is wet. When the hair is dry, more space exists between the hairs, thus more light penetrates and the color appears brighter and more luminous.
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See also:
How hair turns gray
Hair colors and hair coloring
The different shades of gray hair
Is wet hair more fragile and more prone to hair loss?