A rotten shade of red (4)

I believe the answer is:

rust

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'shade of red' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for rust that I've seen before include "Reddish-brown coating formed on iron by moisture" , "Reddish coating formed on iron through 11 across" , "Reddish-brown coating formed on metal by damp" , "Reddish oxide coating on iron" , "deteriorate" .)

I've seen this clue in the Evening Standard.

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