Mark the colour of the blocks here for printing. (6)
I believe the answer is:
inkjet
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for inkjet that I've seen before include "and 20 Across: Type of computer accessory" , "Type of office printer" , "Printer powered by a liquid engine" , "Type of electronic printing" , "Kind of printer" .)