whiteness vs off-white

whiteness and off-white both are nouns.

Word	Noun	Adjective	Verb	Adverb
whiteness	Yes	No	No	No
off-white	Yes	Yes	No	No

As nouns, off-white is a hyponym of whiteness; that is, off-white is a word with a more specific, narrower meaning than whiteness:

whiteness: the quality or state of the achromatic color of greatest lightness (bearing the least resemblance to black)
off-white: a shade of white the color of bleached bones

Other hyponyms of whiteness include alabaster, bleach, bone, ivory, pearl, chalk, frostiness, hoariness.

whiteness (noun)	off-white (noun)
the quality or state of the achromatic color of greatest lightness (bearing the least resemblance to black)	a shade of white the color of bleached bones
lightness or fairness of complexion
the state of being unsullied by sin or moral wrong; lacking a knowledge of evil

whiteness (adjective)	off-white (adjective)
	of something having a color tending toward white