Many substances are added to glass to change it from clear to many other colors, from the colors of the rainbow to every color in between. Often times, metal or metallic compounds are used to change the colors. Colors often vary or change according to temperature, concentration, amount of oxidization, as well as many other factors. Many Glass Artisits, like me, buy the glass pre-colored, in sheet or rod form. Sometimes the colors do change when you reheat and cool them, so you should be aware of whether or not the glass will change.
Compounds and the colors they make in glass:
Iron Oxides---Greens, Browns
Maganese Oxides---Deep Amber, Amethyst
Colbalt Oxides--- Deep Blue
Gold Chloride---Ruby Red
Selenium Compounds---Reds
Carbon Oxides---Amber/Brown
Magamese, Colbalt, & Iron---Black(Deep Purple)
Antimony Oxides---White
Uranium Oxides---Yellow green (it glows!)
Sulfur Compunds---Amber/Brown
Copper Compounds---Light Blue, Red
Tin Compounds---White
Lead With Antimony---Yellow
