Colouring agent (3)

I think it could be either:

Which length description are you looking for?:

dye

'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" .)

I've seen this clue in The Times, The Telegraph and The Sunday Times.

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