achromatic colour vs colouring

achromatic colour and colouring both are nouns.

Word	Noun	Adjective	Verb	Adverb
achromatic colour	Yes	No	No	No
colouring	Yes	No	No	No

As nouns, colouring is a hypernym of achromatic colour; that is, colouring is a word with a broader meaning than achromatic colour:

achromatic colour: a color lacking hue; white or grey or black
colouring: a visual attribute of things that results from the light they emit or transmit or reflect

Other hypernyms of achromatic colour include color, coloring, colour.

achromatic colour (noun)	colouring (noun)
a color lacking hue; white or grey or black	the act or process of changing the color of something
	a visual attribute of things that results from the light they emit or transmit or reflect
	a digestible substance used to give color to food