Printing ink: no source of colour used up (4)
I believe the answer is:
cyan
'printing ink' is the definition.
(one of the main inks used in printing)
'no source of colour used up' is the wordplay.
'no' becomes 'nay' (archaic form 'no').
'source of' indicates taking the first letters.
'used up' says the letters should be written in reverse (in a down clue, letters go up).
The initial letter of 'colour' is 'c'.
'nay'+'c'='nayc'
'nayc' reversed gives 'CYAN'.
(Other definitions for cyan that I've seen before include "Printer's blue" , "Printer's colour" , "Blue printing ink" , "Bluish-green" , "Greenish blue" .)