brightness vs luster

brightness and luster both are nouns.

Word	Noun	Adjective	Verb	Adverb
brightness	Yes	No	No	No
luster	Yes	No	No	No

As nouns, luster is a hyponym of brightness; that is, luster is a word with a more specific, narrower meaning than brightness:

brightness: the location of a visual perception along a continuum from black to white
luster: a quality that outshines the usual

brightness (noun)	luster (noun)
the location of a visual perception along a continuum from black to white	a surface coating for ceramics or porcelain
the quality of being luminous; emitting or reflecting light	a quality that outshines the usual
intelligence as manifested in being quick and witty	the visual property of something that shines with reflected light