yellow vs yellowed

yellow is a noun but yellowed is not a noun.

Word	Noun	Adjective	Verb	Adverb
yellow	Yes	Yes	Yes	No
yellowed	No	Yes	No	No

As adjectives, yellow and yellowed are synonyms defined as:

yellow (noun)	yellowed (noun)
yellow color or pigment; the chromatic color resembling the hue of sunflowers or ripe lemons

yellow (adjective)	yellowed (adjective)
of the color intermediate between green and orange in the color spectrum; of something resembling the color of an egg yolk	changed to a yellowish color by age
easily frightened
cowardly or treacherous
changed to a yellowish color by age
typical of tabloids
affected by jaundice which causes yellowing of skin etc

yellow (verb)	yellowed (verb)
turn yellow