Why do clothes look dark when they immersed in the water

An object made up of several layers interact differently with the light and then the overall perception of colour will be dependent on the overall nature of light received by the eyes from the object. If you have some cotton fabric there are indeed two such layers, one of the loose surface fibers and the second one of the actual surface of cloth. The upper layer doesn't absorb significantly waves of any particular wavelengths, they only scatter the light falling on them. When this scattered light adds up to the reflected light from the opaque object, the result is the perception of a lighter tint. When the layer is removed, like if the cloth is soaked in water, the color is perceived to be darker.