Mark to allow a colour (7)
I believe the answer is:
scarlet
'a colour' is the definition.
(I know that scarlet is a type of red)
'mark to allow' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'to allow' becomes 'let' ('let' can be a synonym of 'allow').
'scar'+'let'='SCARLET'
(Other definitions for scarlet that I've seen before include "immoral" , "Cherry-red" , "Bright red of clarets" , "Fallen" , "Shade of red" .)