grey vs color in

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

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

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

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

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