Why Does My Hair Look Dark When Wet?
When your hair gets wet, the water makes your individual hairs stick close together. The water also forms a thin film on your hair. Both of these occurrences affect the way light bounces off of your wet hair.
Rather than reflecting the same amount of light back to your eyes, wet hair reflects less light back to your eyes. Since your hairs are closer together when wet, they tend to trap and absorb more light.
In addition, light that does get reflected back often hits the inside of the thin water film and gets reflected back into your hair, where more light gets absorbed.
When more light is absorbed by your wet hair, less light gets reflected back to your eyes. The result is that your hair appears darker than when it’s dry. Your hair itself doesn’t change, though. All you have to do is let it dry and it’ll be back to its usual color!