Color in glass may be obtained by addition of electrically charged ions (or color centers) that are homogeneously distributed, and by precipitation of finely dispersed particles.
Ordinary soda-lime glass appears colorless to the naked eye when it is thin. Iron(II) oxide (FeO) impurities of up to 0.1 wt% produce a green tint, which can be viewed in thick pieces or with the aid of scientific instruments.
Further FeO and chromium(III) oxide (Cr2O3) additions may be used for the production of green bottles. Sulfur, together with carbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black.
A glass melt can also acquire an amber color from a reducing combustion atmosphere.