tarnish vs discolouration

tarnish and discolouration both are nouns.

Word	Noun	Adjective	Verb	Adverb
tarnish	Yes	No	Yes	No
discolouration	Yes	No	No	No

As nouns, discolouration is a hypernym of tarnish; that is, discolouration is a word with a broader meaning than tarnish:

Other hypernyms of tarnish include discoloration, stain.

tarnish (noun)	discolouration (noun)
discoloration of metal surface caused by oxidation	the act of changing the natural color of something by making it duller or dingier or unnatural or faded
	a soiled or discolored appearance

tarnish (verb)	discolouration (verb)
make dirty or spotty, as by exposure to air; also used metaphorically