Ghost is another name given to hypo or hypomelanistic BPs.
I was talking to Adam at 8ball pythons, and he said that a few years ago, breeders were under the impression if you bred an orange ghost to an orange ghost you only get more orange ghosts, now, they think that you can actually get butterscotch ghosts, orange ghosts and yellow ghosts out of an orange to orange breeding.
I've done no breeding, so i cant give my opinion on this, but i thought i'd pass along the info he'd given me.
To go back to the original question, from what i have heard....:
Orange, butterscotch and yellow ghosts are compatible through eachother.
Green Ghosts, im not sure about.
Blue ghosts are only compatible with other blue ghosts.