yellow green vs yellowish green

yellow green and yellowish green both are nouns.

Word	Noun	Adjective	Verb	Adverb
yellow green	Yes	No	No	No
yellowish green	Yes	No	No	No

As nouns, yellow green and yellowish green are synonyms defined as:

Other synonyms of yellow green include chartreuse, Paris green, pea green.

yellow green (noun)	yellowish green (noun)
a shade of green tinged with yellow	a shade of green tinged with yellow