On forehead, new colour (5)
I believe the answer is:
brown
'colour' is the definition.
(brown is a colour)
'on forehead new' is the wordplay.
'on' means one lot of letters go next to another.
'forehead' becomes 'brow' (synonyms).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'brow' next to 'n' is 'BROWN'.
(Other definitions for brown that I've seen before include "Colour; ex-PM" , "cook to a rich colour?" , "Dark - tanned - successor to Blair?" , "Colour of earth" , "bookish father" .)