Phenomenon in which porcelain appears different under varying light condition is:

Metamerism Phenomenon in which the color of an object under one type of light source appears to change when illuminated by a different light source. Light rays can also be dispersed, giving a color or shade that varies in different teeth. The dispersion can vary with the wavelength of the light.  Therefore, the appearance of the teeth may vary according to whether they are viewed in direct sunlight, reflected daylight, tungsten light, or fluorescent light. This phenomenon is called metamerism. Ref:Phillip’s Science of Dental Materials ed 12 pg 464