The thin colored layer fused onto clear or differently colored glass is called flashed glass. It allows a glazier to use a surface layer of strong color over a paler body of glass.
Flashed glass is made by dipping a gather of molten glass into another molten glass of a contrasting color and then blowing or forming the combined material. The colored layer is usually much thinner than the underlying glass.
A craftsman can selectively abrade the surface with a wheel or acid to expose the color beneath. This technique creates details such as lettering, heraldic patterns, and two-tone garments without requiring a separate piece for every color.
Flashed glass is not the same as opalescent glass, which describes a milky or internally varied visual effect. It is also different from cast glass, made by casting molten glass into a mold or onto a surface.