Why do clothes look dark when immersed in water?

The colour of an object is determined by the spectral composition of the light reflected by it. When a surface is dry, the reflected light rays corresponding to the colour of the surface are superposed with diffused white light arising out of random scattering resulting from the roughness and irregularities of the surface. In the case of clothes, this superposition tends to fade the reflected colour.

 If clothes are immersed in water, all the kinks and interspaces are filled with water. This minimizes scattering of light on the surface. So the reflected light comes out unsuppressed with the basic hue of the cloth. Hence it looks richer and darker than when dry.

 The effect is conspicuous on cotton clothes as the fibres are loosely packed with a lot of microscopic air spaces. This increases the surface area and consequently the scattering of light. Hence cotton clothes look light when dry and dark when wet. The effect is not so much in synthetic and silk clothes as their surfaces are smoother and very little water is absorbed by them.