Ape is changing colour (5)
I believe the answer is:
sepia
'colour' is the definition.
(I know that sepia is a type of pigment)
'ape is changing' is the wordplay.
'changing' is an anagram indicator.
'ape'+'is'='apeis'
'apeis' anagrammed gives 'SEPIA'.
(Other definitions for sepia that I've seen before include "source of ink" , "Sort of drawing" , "Former photographic pigment" , "Reddish-brown colour as in old photos" , "Brownish tone associated with old photographs" .)