light brown vs ecru

light brown and ecru both are nouns.

Word	Noun	Adjective	Verb	Adverb
light brown	Yes	No	No	No
ecru	Yes	No	No	No

As nouns, ecru is a hyponym of light brown; that is, ecru is a word with a more specific, narrower meaning than light brown:

Other hyponyms of light brown include tan, topaz, dun, fawn, grayish brown, greyish brown, beige.

light brown (noun)	ecru (noun)
a brown that is light but unsaturated	a very light brown