Crossword Genius

Colour of river as rendered by cockney? (5)

Ross

I believe the answer is:

umber

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for umber that I've seen before include "Dark to reddish brown" , "Dark brown coloring matter" , "Brown earthy substance used as pigment" , "Natural colouring matter, dark brown" , "Dark brown (pigment)" .)

I've seen this clue in The Times.
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