grey vs colour in

grey is a noun but colour in is not a noun.

Word	Noun	Adjective	Verb	Adverb
grey	Yes	Yes	Yes	No
colour in	No	No	Yes	No

As verbs, colour in is a hypernym of grey; that is, colour in is a word with a broader meaning than grey:

Other hypernyms of grey include color, color in, colorise, colorize, colour, colourise, colourize.

grey (noun)	colour in (noun)
horse of a light gray or whitish color
clothing that is a grey color
a neutral achromatic color midway between white and black
any organization or party whose uniforms or badges are grey

grey (adjective)	colour in (adjective)
of an achromatic color of any lightness intermediate between the extremes of white and black
intermediate in character or position
showing characteristics of age, especially having grey or white hair
used to signify the Confederate forces in the American Civil War (who wore grey uniforms)

grey (verb)	colour in (verb)
turn grey	add color to
make grey