red region vs nether region

red region and nether region both are nouns.

Word	Noun	Adjective	Verb	Adverb
red region	Yes	No	No	No
nether region	Yes	No	No	No

As nouns, nether region is a hypernym of red region; that is, nether region is a word with a broader meaning than red region:

red region: a place of eternal fire envisaged as punishment for the damned
nether region: (Christianity) the abode of Satan and the forces of evil; where sinners suffer eternal punishment

Other hypernyms of red region include Hell, infernal region, Inferno, perdition, pit.

red region (noun)	nether region (noun)
a place of eternal fire envisaged as punishment for the damned	(Christianity) the abode of Satan and the forces of evil; where sinners suffer eternal punishment