Turns blue? (4)
I believe the answer is:
dyes
'turns blue?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dyes that I've seen before include "turns green, possibly" , "Substances for staining or colouring" , "Colours (fabric)" , "Colouring materials" , "Stains" .)