Printer's blue (4)
I believe the answer is:
cyan
'printer's blue' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cyan that I've seen before include "Greenish blue" , "Primary subtractive colour" , "Greenish-blue colour complementary to red" , "Printer's colour" , "Blue printing ink" .)