The fading process is similar to car paint. ie it is due to microscopic wear and tear thereby reducing the smoothness of the surface. The less smooth the surface the less shine. This is due to the inability to reflect light. Hence, on the same token, the shine and quality of our gold plating will depend on the original condition of the item plated.
Specular surface are very smooth surface. When the wavelength of light is greater than the surface imperfections, the result is specular reflection. When more light ray falls onto the eyes, the eyes perceived a more shiny surface. Some example of specular reflections are mirror, finely polished stainless steel chromed surface, calm water surface
Rough surface causes diffuse reflection. Hence when less light falls onto your eyes, the eyes perceived a less shiny object. eg in the case of 24k gold-plating or even car paintwork, after repeated washing, tiny microscopic scratches forms on the surface causing diffuse reflection which results in a less shiny appearance(ie fading)
Most surfaces are a combination of specular and diffuse reflection. When the smooth surface reflects the full spectrum of light, we see white light. When a surface absorb most lights and reflects only red and some spectrum of blue light(example above) we see the surface as "red". Hence the color of gold is a reflection of the combination of the electromagnetic light spectrum that combines to give the gold color. The brightness(ie shiny) of the gold surface then depends on how smooth the surface is.
