Colouring agent (3)
'colouring agent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dye that I've seen before include "Pigment" , "Colouring; sounds like fade" , "Colourant" , "Colouring liquid" , "Staining material" .)
(Other definitions for pigment that I've seen before include "Colour or dye" , "Colouring material" , "Colourful substance" , "become imbued" , "Colouring matter" .)