blotch vs colour in

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

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

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

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

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

blotch (noun)	colour in (noun)
an irregularly shaped spot

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