Without this feature, different pieces of glass could not be fused together, they would shatter. Adding different metal oxides to sand, soda and lime, the basic formula for glass creates different colors. Light blues require copper, while dark blues require very small quantities of cobalt. Amethyst glass contains manganese. Opaque white can contain either tin or calcium.
Selenium is one metal oxide that is used to produce reddish colors.
The information on color in glass is derived from the web page on colors at the Corning Museum of Glass web site. The finished product has more than one color, and the different colors can be seen when the glass is viewed from different angles. If the base glass is black, however, there is no transmitted color to be seen. In order to be fusible, the base glass must be tested compatible. Due to the layers of metal oxides, this glass is stiffer than other fusible glass and so requires special handling.