Colors are a result of different dyes in the cloth or material absorbing and then reflecting different colors of light. Water also absorbs and reflects light, so it changes how we see the wet spot on the material.

A coating of water reduces the roughness of the surface. The rougher a surface, the more reflective it will be, so smoothing it makes it less reflective, i.e. darker.

Actually, it's still plenty reflective, but only at an angle opposite that of the incoming light (basically, like a mirror). The light just gets bounced off in a focused beam instead of being scattered.