fellow worker vs colleague

fellow worker and colleague both are nouns.

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

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

Other synonyms of fellow worker include co-worker, workfellow.

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