Colour of ruddy exterior (3)
I believe the answer is:
dye
'colour' is the definition.
(I know that dye is a type of colour)
'of ruddy exterior' is the wordplay.
'of' indicates the answer is hidden within the clue.
'DYE' can be found hidden inside 'ruddy exterior'.
(Other definitions for dye that I've seen before include "Liquid used to change colour of clothes" , "Staining material" , "Colour or stain something" , "Colour by dipping in liquid" , "Tint" .)