Can be green, blue, or sepia externally (3)
I believe the answer is:
sea
'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" .)