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What is Greening refers to: and explain in detail?
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