Nominally getting louder on turning red (3)

I believe the answer is:

lou

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'red' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?

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

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(Other definitions for lou that I've seen before include "-- Costello, Abbott's partner" , "Male or female" , "man" , "--- Costello, US comic" , "-- Reed, rock musician" .)

I've seen this clue in the Evening Standard.

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