Black, White and Grey. Never heard of Grey magick. The black and white refer to intent.
Black is generally connotated with the negative aspect, while White is connotated with positive aspects.
However, you cannot have one without the other. Some practitioners strictly follow the "harm none your will be done". But, when thinking this way, if for example, you cast to get that great job for yourself, in retrospect, someone else is not going to get that job and may suffer from it.
For every action there is a reaction.
To cast against a child molester to be impotent would be considered harming someone, however, in retrospect again it would benefit the potential child victims than he may no longer be capable to prey upon.
While we none know the truth of universal justice, asking for guidance in any endeavor is wise.
Belief is to power what knowledge is to skill, if you have both, you should be fine.