Greenish-blue colour complementary to red (4)
I believe the answer is:
cyan
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greenish-blue colour complementary to red</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 cyan that I've seen before include "Printer's colour" , "Printer's blue" , "Bluish-green" , "Blue hue" , "Greenish blue" .)