monotone vs monotonous

monotone is a noun but monotonous is not a noun.

Word	Noun	Adjective	Verb	Adverb
monotone	Yes	Yes	No	No
monotonous	No	Yes	No	No

As adjectives, monotone and monotonous are synonyms defined as:

Other synonyms of monotone include flat, monotonic.

monotone (noun)	monotonous (noun)
an unchanging intonation
a single tone repeated with different words or different rhythms (especially in rendering liturgical texts)

monotone (adjective)	monotonous (adjective)
of a sequence or function; consistently increasing and never decreasing or consistently decreasing and never increasing in value	tediously repetitious or lacking in variety
sounded or spoken in a tone unvarying in pitch	sounded or spoken in a tone unvarying in pitch