Greening refers 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.”

Ref: Phillip’s Science of Dental Materials ed 12 pg 380
Greenish discolouration of porcelain due to escaping of silver vapours
Also known as wet strength of gypsum product
Observed in silica bonded investments due to drying of colloidal silica gel
All of the above