colleague vs fellow worker

colleague and fellow worker both are nouns.

Word	Noun	Adjective	Verb	Adverb
colleague	Yes	No	No	No
fellow worker	Yes	No	No	No

As nouns, colleague and fellow worker are synonyms defined as:

Other synonyms of colleague include co-worker, workfellow.

colleague (noun)	fellow worker (noun)
an associate that one works with	an associate that one works with
a person who is member of one's class or profession