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 reelected colour.
If clothes are immersed in water, all the kinks and interspaces are filled with
water. This minimises 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 fibers 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.