Paint that's aquamarine, say (11)

I believe the answer is:

watercolour

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'paint' is the definition.
(I know that paint is a is a more specific form of the action of watercolour)

'that's aquamarine say' is the wordplay.
'that' becomes 'water' (I can't explain this - if you can you should give a lot more credence to this answer).
'aquamarine say' becomes 'colour' (aquamarine is a kind of colour).
'water'+'colour'='WATERCOLOUR'

Can you help me to learn more?

(Other definitions for watercolour that I've seen before include "Type of painting" , "Painted picture" .)

I've seen this clue in The Mirror.

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