mottle vs color in

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

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

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

mottle: mark with spots or blotches of different color or shades of color as if stained
color in: add color to

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

mottle (noun)	color in (noun)
an irregular arrangement of patches of color

mottle (verb)	color in (verb)
colour with streaks or blotches of different shades	add color to
mark with spots or blotches of different color or shades of color as if stained