Deliberate coloration of glass requires the addition of an appropriate mineral, e.g. malachite for green, pyrolusite for purple, etc. The crushed mineral and crushed frit [see Glassmaking] are mixed together and then heated through to the point of fusion.(Often, these mixtures have a lower fusion point than naturally colored glass.)
The glass's color, and its intensity, are dependent upon the furnace atmosphere during firing. The usual, oxygen-rich environment (i.e. an oxidizing state) will produce one color while an oxygen-starved environment (i.e. a reducing state) will produce another. For example, the iron impurities that usually color glass aquablue or light green will produce an amber shade when fused in a reducing environment.