Add color to (4)
I believe the answer is:
tint
'add color to' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tint that I've seen before include "work for coiffeuse?" , "Hue, shade" , "A slight shade of colour" , "(Softer variety of) colour" , "Hair dye" .)