Colour I associate with wrongdoing (6)

I believe the answer is:

indigo

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'colour' is the definition.
(I know that indigo is a type of dye)

I cannot understand how the rest of the clue works.

(Other definitions for indigo that I've seen before include "Ellington's Mood" , "type of dye" , "Dark blue dye (plant)" , "Blue-violet colour" , "Blue-purple colour in spectrum" .)

I've seen this clue in The Globe and Mail.

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