When gold fused with porcelain is fired at very high temperature, greenish discoloration of cervical margin is due to:
Discoloration of the porcelain near the cervical region of the metal-ceramic prosthesis has been reported to occur when a silver-containing alloy is used as the substrate. Color changes included green, yellow-green, yellow-orange, orange and brown hues. This discoloration  phenomenon has generally been called “greening.” It was likely caused by the colloidal dispersion of silver atoms entering body and incisal porcelain or the glazed surface from vapor transport or surface diffusion.
The extent of porcelain discoloration was most severe for higher-silver-content alloys, lighter shades, multiple firing procedures, higher temperatures, body porcelain in direct contact with the alloy, vacuum firing cycles and with certain porcelains containing lower opacifier and higher sodium contents.
Silver is added to Au-Ag-Cu casting alloys to offset the reddish hue contributed by Cu.
Ref: Phillips 12 ed page no 413
High firing temperature
Disintegration of opaque layer
Presence of silver
Formation of excess oxides