Colour of river in 'ull (5)

I believe the answer is:

umber

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'colour' is the definition.
(umber is a kind of colour)

I can't explain the remainder of the clue.

(Other definitions for umber that I've seen before include "Natural pigment" , "Deep brown" , "Brown earthy substance used as pigment" , "Usually dark brown mineral" , "Yellow-brown pigment" .)

I've seen this clue in The Independent.

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