Briefly explain what determines the characteristic color of (a) a metal and (b) a transparent nonmetal.

A) The colour of a metal is often obtained by the wavelength distribution scale (which is explained below). The radiation of light which the metal fails to absorb, gets reflected. So, the unabsorbed light which is reflected back is visible to us. Hence, we see the color of a metal.

b) A transparent colour of non metal is seen by us, because of the radiant incident ray of light is unabsorbed by the material and is transmitted through it. So, in this case there is no absorption or reflection taking place and hence there is no colour observed.

The colour of any element usually comes from the electron charge transfer between the lower energy to higher energy level by absorption of the ray of light.

Also there are other reasons too, as in organic compounds, the colour is obtained by hyper conjugation which means there is less gap between the higher molecular orbital and lower molecular orbital. So, the absorption of light takes place in these compounds.

The other reason for absorption of light and imparting colours into its substrate is because of the Wavelength distribution. On the wavelength scale, the wavelength of colour that is absorbed is never seen as the same to us, instead we see the complementary colour of it. Example, if we see a red colour, then the colour absorbed by the substrate should be blue.