Why does the colour of gold sol change with particle size?
The color of conductive nanoparticles depends on plasmonics. In brief, quantum behavior predominates in the few electrons of a nanoparticle, rather than as a continuous conduction band. Surface plasmons strongly influence the color and polarization of light in sols and colloids. For example, for hundreds of years, stained glass has been made with gold or selenium to get pinks and reds.