Go dark? (3)
I believe the answer is:
dye
'go dark?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dye that I've seen before include "Colour (fabrics)" , "Pigment" , "Colourant" , "Colouring; sounds like fade" , "Colour by dipping in liquid" .)