Question

Why does wet clothes appear to be darker than dry clothes?

Answer 1

A wet cloth looks darker because less light is reflected from a wet cloth. Any cloth is woven from a yarn or fibre. That fibre is in turn made of smaller micro-fibres. Light comes from the room lights, or from the Sun, and lands on the cloth. Some of the photons of light are absorbed, but some are reflected and land on your retina - and that gives you the sensation of seeing the cloth as having a certain level of brightness. But when the cloth gets wet, the water fills in the gaps between each individual strand of fibre, and also between each individual micro-fibre. When light falls on the wet cloth, some of it is now more likely to enter the water, and be bent away from your eyes. So some of the light that would have previously been reflected off the cloth back to your eyes, is now bent away.

Fewer photons of light get back to your eyeball, and so the wet cloth "appears" darker than the dry cloth. But as the water gradually evaporates, more and more light is reflected back to your eyeball, and you see the brighter colour of the fabric again.