Mark to allow colour (7)
I believe the answer is:
scarlet
'colour' is the definition.
(scarlet is a kind of colour)
'mark to allow' is the wordplay.
'mark' becomes 'scar' ('scar' can be a synonym of 'mark'**).
'to allow' becomes 'let' ('let' can be a synonym of 'allow').
'scar'+'let'='SCARLET'
(Other definitions for scarlet that I've seen before include "Vivid red" , "bloody?" , "Bright red of clarets" , "Shade of red" , "Brilliant red colour" .)