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Type of trout is naturally colourful (7)

Ross

I believe the answer is:

rainbow

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'type of trout' is the definition.
(I've seen this before)

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

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(Other definitions for rainbow that I've seen before include "Natural arch of colours" , "Colourful arc in the sky" , "Heavenly phenomenon" , "Spectral display" , "Colourful display" .)

I've seen this clue in The Globe and Mail.
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