Greenish-blue colour complementary to red (4)
I believe the answer is:
cyan
'greenish-blue colour complementary to red' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cyan that I've seen before include "Printer's colour" , "Printer's blue" , "Bluish-green" , "Blue hue" , "Greenish blue" .)