When something gets wet, it usually appears darker. Why?
We see an object because light is reflecting off it to our eyes. The light surfaces reflect light better than the dark surfaces. When light falls on the surface of the object, some of the light is reflected by the surface, some may be absorbed within the object while some may pass through the object. The light bends when it goes from one material to another. We call this bending refraction. The objects with high index of refraction are reflecting more light and looks lighter. The water has an intermediate index of refraction, between air and many solid materials. So, the water is reflecting less light than the other objects such as cloth, wood or stone. The water reduces the reflection, allowing more light to be absorbed by the object. Finally, the wet object looks darker.
Image By Neva Swensen