true fir vs silver fir

true fir and silver fir both are nouns.

Word	Noun	Adjective	Verb	Adverb
true fir	Yes	No	No	No
silver fir	Yes	No	No	No

As nouns, silver fir is a hyponym of true fir; that is, silver fir is a word with a more specific, narrower meaning than true fir:

true fir: any of various evergreen trees of the genus Abies; chiefly of upland areas
silver fir: any of various true firs having leaves white or silvery white beneath

Other hyponyms of true fir include Abies bracteata, Abies venusta, bristlecone fir, Santa Lucia fir.

true fir (noun)	silver fir (noun)
any of various evergreen trees of the genus Abies; chiefly of upland areas	any of various true firs having leaves white or silvery white beneath