Nicole Kidman - cranberry juice
Nicole is rumoured to use cranberry juice to enhance her natural colour.
To try this at home buy fresh cranberries and blend them into a paste. Shampoo and condition your hair before pouring the cranberry mix all over it. Work into the ends then rinse with cool water.
While it is not a well documented treatment, cranberry juice contains tannins which can add a coppery tone to hair.