Metallic bonding is considered as metal ions embedded in a 'sea" of electrons. In a metal nanoparticles, the surface electrons are oscillate against the electric force field created by the positively charged metal atom nuclei. When light is incident on the metal nanoparticles, a resonance condition is matched when the frequency of the incident light matches the frequency of surface electrons oscillating due to the force field of the nuclei. This is called the Surface Plasmon Resonance it depends on the size and shape of the metal nanoparticles. It is clearly seen in the visible range for silver and gold nanoparticles. If you decrease the size of gold nanoparticles from 100 nm down to 1 nm you can get various colours due to surface plasmon resonce. So is for Ag nanoparticles.

thanks but I say about organic coating aspetial paint.

but you say metalic coating.but thanks