Artificially coloured (4)
I believe the answer is:
dyed
'artificially coloured' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dyed that I've seen before include "Artificially coloured" , "Changed to a new colour" , "(Of fabric) coloured" , "Imbued with artificual colour" , "Cloth is often" .)