day lily vs lemon lily

day lily and lemon lily both are nouns.

Word	Noun	Adjective	Verb	Adverb
day lily	Yes	No	No	No
lemon lily	Yes	No	No	No

As nouns, lemon lily is a hyponym of day lily; that is, lemon lily is a word with a more specific, narrower meaning than day lily:

day lily: any of numerous perennials having tuberous roots and long narrow bladelike leaves and usually yellow lily-like flowers that bloom for only a day
lemon lily: a day lily with yellow flowers

Other hyponyms of day lily include Hemerocallis flava, Hemerocallis lilio-asphodelus.

day lily (noun)	lemon lily (noun)
any of numerous perennials having tuberous roots and long narrow bladelike leaves and usually yellow lily-like flowers that bloom for only a day	a day lily with yellow flowers
any of numerous perennials having mounds of sumptuous broad ribbed leaves and clusters of white, blue, or lilac flowers; used as ground cover