I don't think ghosts have to look white or green or anything.
I think they just looks like us, but.. somehow you know or notice that it's not just a human, but a ghost instead.
However, in fact I think it doesn't make ANY sense that ghosts looks like humans.
I mean:
Imagine you believe in ghosts.
That would make us all have a soul/ghosts in our bodies.
But .. why would your ghosts looks like your body?


