somber vs melancholy

somber is not a noun while melancholy is a noun.

Word	Noun	Adjective	Verb	Adverb
somber	No	Yes	No	No
melancholy	Yes	Yes	No	No

As adjectives, somber and melancholy are synonyms defined as:

Other synonyms of somber include sombre.

somber (noun)	melancholy (noun)
	a humor that was once believed to be secreted by the kidneys or spleen and to cause sadness and melancholy
	a feeling of thoughtful sadness
	a constitutional tendency to be gloomy and depressed

somber (adjective)	melancholy (adjective)
lacking brightness or color; dull	grave or even gloomy in character
grave or even gloomy in character	characterized by or causing or expressing sadness