They change colours (4)
I believe the answer is:
dyes
'colours' is the definition.
(dyeing is a kind of colouring)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dyes that I've seen before include "Colours (fabric)" , "Substances for staining or colouring" , "Stains" , "turns green, possibly" , "Colour" .)