Hair colour change after death

My grandmother died at the age of seventy - nine three years ago. When she was buried her hair was white. It is obligatory in Greece ,where I live ,that the dead should be exhumated after three years of their burial. Last Monday, 11 July, 2005 after the macabre scene of the exhumation my mother and I were shocked to see that my grandmother's hair colour had changed from white to a dark shade of blonde,in particular, the shade of her favourite hair dye she used when she had her hair dyed! Unbelievable as it may sound it is very much true!
My grandmother stopped having her hair died approximately four years before she passed away. On the day of her death there was no trace of hair dye on her head. She was on medication for Alzheimer, she also took tranquilisers because during the period of her illness (a year of rapid deterioration) she would scream and shout and be extrermely restless at nights. She also had to go through six chemotherapies for breast cancer a year or so before she died.
I would appreciate it if you could tell me something about the possible explanations of how the hair colour change may have taken place because I guess that hairdressers are really hard to find in one's coffin when this has been laid under! Moreover, my grandmother suffered a terrible end and I'm really wondering whether the hair dyes she had used for years had messed with her head and had anything to do with her being reduced to a "vegetable" state. I was really scared to see this colour change because I'm 37 years old and I have my hair dyed, too. My husband keeps telling me not to as he really worries about the consequences...
Thanks,
Eleni.