Amend the colour, obscuring old line? (7)
I believe the answer is:
retouch
'amend' is the definition.
I don't understand how the rest of the clue works.
(Other definitions for retouch that I've seen before include "Repair a painting" , "Correct painting" , "Paint over" , "Improve a photo, with a pencil say" , "Fine-tune" .)