It changes colour (3)
I believe the answer is:
dye
This might be a double definition.
'it changes' is the first definition.
The definition suggests a singular noun which matches the answer.
'colour' is the second definition.
(I know that dye is a more specific form of the action colour)
Can you help me to learn more?
(Other definitions for dye that I've seen before include "Hair colouring" , "Colouring agent" , "Colourant" , "Liquid used to change colour of clothes" , "Colour or stain something" .)