Deep blue pigment (11)
I believe the answer is:
ultramarine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deep blue pigment</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ultramarine that I've seen before include "See a colour" , "Vivid blue" , "Blue pigment originally made from lapis lazuli" , "Deep blue pigment" , "Brilliant deep blue" .)