noun

the process of becoming discoloured, or the state of being discoloured.

teeth with amalgam fillings may run the risk of discoloration

stain,mark,patch,soiling,streak,spot,blotch,tarnishing,blemish,flaw,defect,disfigurement,bruise,contusion,birthmark,liver spot,age spot,splodge,splotch,ecchymosis,naevus