old gold vs yellowness

old gold and yellowness both are nouns.

Word	Noun	Adjective	Verb	Adverb
old gold	Yes	No	No	No
yellowness	Yes	No	No	No

As nouns, yellowness is a hypernym of old gold; that is, yellowness is a word with a broader meaning than old gold:

old gold: a dark yellow
yellowness: yellow color or pigment; the chromatic color resembling the hue of sunflowers or ripe lemons

Other hypernyms of old gold include yellow.

old gold (noun)	yellowness (noun)
a dark yellow	yellow color or pigment; the chromatic color resembling the hue of sunflowers or ripe lemons