skin colour vs darkness

skin colour and darkness both are nouns.

Word	Noun	Adjective	Verb	Adverb
skin colour	Yes	No	No	No
darkness	Yes	No	No	No

As nouns, darkness is a hyponym of skin colour; that is, darkness is a word with a more specific, narrower meaning than skin colour:

skin colour (noun)	darkness (noun)
the coloring of a person's face	a swarthy complexion
	having a dark or somber color
	an unenlightened state
	an unilluminated area
	absence of light or illumination
	absence of moral or spiritual values