Short answer: wetting a rough surface changes the path of light. Water replaces many air-filled gaps, reducing some of the scattering that normally sends light back toward your eyes. More light can travel into the material, where some is absorbed, so the surface often looks darker.
Dry surfaces are tiny light scatterers
Concrete, cloth, soil and asphalt are rough on a microscopic scale. A dry surface contains many boundaries between solid material and air. Incoming light bounces and scatters at those boundaries, and a portion returns to you as diffuse reflection.
Add water and the optical mismatch changes. Because water's refractive index is closer to many solid materials than air's is, light can pass into the surface more readily. Light that tries to come back out can also be redirected at the water-air boundary, giving the material another chance to absorb it.
Then why can a wet road also glare?
A smooth film of water produces specular reflection—mirror-like glare. So the same wet road can look darker overall while showing a bright reflection of the sky or headlights at the right angle. “Darker” does not mean the water dyed the material; it means the direction and amount of returning light changed.
Quick follow-ups
FAQ
Does water actually change the color of the material?
Usually no. It changes how light is reflected, scattered and absorbed, which changes the color and brightness that your eyes perceive.
Why can wet pavement look shiny and dark at the same time?
The rough surface can return less diffuse light while the smoother water layer creates bright, directional reflections.
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