Mate pointing out natural colouring (12)
I believe the answer is:
pigmentation
'natural colouring' is the definition.
(I've seen this before)
'mate pointing out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'mate'+'pointing'='matepointing'
'matepointing' is an anagram of 'PIGMENTATION'.
(Other definitions for pigmentation that I've seen before include "For example, madder" , "Natural colouring" .)