The colour of any surface is determined by the spectral composition of the light rays reflected by it. When the surface is dry, the rays of light corresponding to the colour of the surface are superimposed with white light that has undergone disordered scattering from the irregularities and unevennesss of the surface (for example, from the fibres of cloth). The presence of this scattering of white light makes the basic colour of the surface paler and less bright.
When the surface is saturated with water, then all the unevenness is smoothed out by the surface film of water, and the scattering of white radiation vanishes. There remains only the basic hue of the surface, which is perceived by us as richer and darker than before the wetting.
