Naturally, categorising things helps newbies to understand them. That is why we use colours they associate with evil, nature, good, etc.
Black = ''Bad'' magick. Anything that manipulates free will.
White = Magick spells for the helping of yourself or others, ''good'' spells.
Grey = A mixture of the two above.
Green = Nature magick. Using a lot of the earth elements, and earth/weather/etc spells.
Of course, there are many other types listed by colours too; but it would take forever to mention them all! Nevertheless, as I said earlier, categorizing makes thinga easier for people to understand. This is why we give each type of magick a label, although they dont have a colour.