Deep violet-blue (6)
I believe the answer is:
indigo
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">deep violet</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">blue</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(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" .)