What is the difference between gray and black magic?

NOT MUCH

A Pagan PerspectiveLet's be very frank here. Magic itself is not white, grey, black, purple or green. It does NOT have a morality of its own. Magic is the manipulation of naturally occurring power by the will of a single practitioner (or a group of like minded practitioners lead by a single will) Magic is power, pure and simple.

To help answer this question I will use electricity as a mundane corollary.

The energy itself is neither good nor bad. It is in its use that polarity can be attributed. When electricity is used to cook food, heat homes and light the darkness it is perceived as good. When it is used to torture people, damage things or kill someone, then it is perceived as bad or evil. The energy itself is neither, it simply is.

In the same way, magical energies are simply there. It is how we draw upon and use the energies that determine how they are perceived. Black, grey, white, purple or green is in the will that directs the energies. Ethics in a practitioner are of paramount importance.