Dirty colour (4)

I believe the answer is:

blue

This clue is a double definition.

'dirty' is the first definition.
(blue can mean pornographic)

'colour' is the second definition.
(blue is a colour)

(Other definitions for blue that I've seen before include "Azure" , "Dismal" , "ball at snooker" , "Glum, sad" , "eg Prussian" .)

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

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