Crossword Genius

Can be green, blue, or sepia externally (3)

Ross

I believe the answer is:

sea

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'can' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see an association between them that I can't see?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for sea that I've seen before include "The main" , "Lesser ocean" , "Cod's home?" , "Marine" , "North, say" .)

I've seen this clue in the Evening Standard.
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