A barney colour (4)

I believe the answer is:

blue

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'colour' is the definition.
(blueing is a kind of colouring)

I don't understand how the remainder of the clue works.

(Other definitions for blue that I've seen before include "Colour of depression" , "Turquoise, say" , "Spend all at once - colour" , "University sportsman - a shade - depressed" , "Down in the mouth" .)

I've seen this clue in the Sydney Morning Herald.

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