melancholy vs sombre

melancholy is a noun but sombre is not a noun.

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

As adjectives, melancholy and sombre are synonyms defined as:

Other synonyms of melancholy include somber.

melancholy (noun)	sombre (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

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