Why are ghosts usually white?

There is a scholarly article written by a professional historian on the subject here:

http://museumofthemacabre.com/research/essays/white-ghosts/

The Museum of the Macabre is great, they have all kinds of ghost related historical stuff!!!

COMMENTS:

Can i just add that i know you said usually white but i thought i would just say that i saw the ghost/spirit of my dead dog when a few days after he died and what i saw was a blur of the colours he was, merged together, not white. i just thought i would share it.