tarnish

nouns

discoloration of metal surface caused by oxidation
tarnish
is a type of (narrower)
a soiled or discolored appearance
"the wine left a dark stain"
discoloration, discolouration, stain

verbs

make dirty or spotty, as by exposure to air; also used metaphorically
"The silver was tarnished by the long exposure to the air"; "Her reputation was sullied after the affair with a married man"
defile, maculate, stain, sully, tarnish