Deep violet-blue (6)
I believe the answer is:
indigo
I believe this clue is a double definition.
'deep violet' is the first definition.
(I've seen this in another clue)
'blue' is the second definition.
(I've seen this before)
(Other definitions for indigo that I've seen before include "Dark blue dye (plant)" , "type of dye" , "Mood - (Duke Ellington)" , "Adulthood" , "supplier of blue stuff" .)