mottle vs colour in

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

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

As verbs, colour in is a hypernym of mottle; that is, colour 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
colour in: add color to

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

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

mottle (verb)	colour 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