Medium colours (4)
I believe the answer is:
oils
'medium colours' is the definition.
The answer and definition can be both related to change as well as being verbs in their -s form.
Maybe you can see an association between them that I can't see?
This is the entire clue.
Can you help me to learn more?
(Other definitions for oils that I've seen before include "Viscous substances kept in silo" , "Well-produced" , "Painting medium" , "Reduces friction" , "Viscous stuffs in silo" .)