Darkens certain colours (6)
I believe the answer is:
shades
'colours' is the definition.
(shade is a kind of colour)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for shades that I've seen before include "suspicions" , "Sunglasses, colloquially" , "Old ghosts or modern sunglasses" , "Cool sunglasses or archaic ghosts" , "Nuances" .)