fellow vs colleague

fellow and colleague both are nouns.

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

As nouns, fellow and colleague are synonyms defined as:

fellow and colleague: a person who is member of one's class or profession

Other synonyms of fellow include confrere.

fellow (noun)	colleague (noun)
a boy or man	an associate that one works with
a man who is the lover of a man or woman	a person who is member of one's class or profession
a friend who is frequently in the company of another
a person who is member of one's class or profession
an informal form of address for a man
a member of a learned society
one of a pair