Colour of mutant (3)
I believe the answer is:
tan
'colour' is the definition.
(I know that tan is a more specific form of the action colour)
'of mutant' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TAN' can be found hidden inside 'mutant'.
(Other definitions for tan that I've seen before include "Tawny" , "Brownish colour" , "Get the effect of the sun." , "Make black and blue" , "Acquire a sunburn" .)