Foundational or subjective characterisation of magic
You have a decision to make.
If you read many fantasy books which use the words "white magic" and "dark magic", and pay attention to how these expressions are used, you'll notice that:
- in some books there is a foundational difference between these two kinds of magic, just like in our world there is a foundational difference between blacksmithing and woodworking; and a very skilled wizard might be able to use both white techniques and dark techniques, just like in our world an artisan is able to hammer an iron nail into a wooden plank;
- in other books there is no foundational difference, and "dark magic" is merely a subjective qualifier given by characters to describe "distasteful" uses of magic: for instance, a wizard who uses magic to murder or enslave other wizards and strengthen his own power will be called a "dark wizard", regardless of the magical techniques he used to achieve his goals.
In the latter option, "dark" really is extremely subjective. If you use magic to murder someone, you'll be called a dark wizard; but if you use magic to murder someone who's already been labeled a dark wizard, then you probably will be called a hero.
Books that make a foundational distinction between white magic and dark magic sometimes do not stop there, and describe more kinds of magic:
- white magic (maybe related to healing, altruism or empathy, or communicating with light creatures like unicorns and angels);
- dark magic (maybe including murder-based rituals, necromancy, and communicating with dark creatures like spiders and demons);
- blood magic (related to bloodlines or involving blood sacrifices, possibly can devise strong targeted curse if you have access to a drop of the victim's blood);
- red magic (related to lust and sex);
- mind or soul magic (related to telepathy);
- elemental magic (related to manipulation of water, fire, lightning, earth, etc.);
Then there is the question of whether the white/dark qualifier applies to the magic technique, or to the magic practitioner. In some books, a skilled sorcerer might be able to use both white magic and dark magic and other kinds of magics; but in some books, a sorcerer who is driven by hate or who relies on too many dark techniques ends up becoming a "dark sorcerer", with some corruption to their soul, able only to use dark techniques and no longer able to use white magic.
Qualifying the source of energy powering the magic
magic works by bending the rules of science but not breaking them
One of the most important laws of physics is conservation of energy.
If sorcerers in your novel can use magic to accomplish great feats, it begs the question of where does the energy for these feats come from? Perhaps what distinguishes between the different kinds of magic is the source of energy. You can use your own energy (which will make you feel hungry/tired/cold and perhaps age prematurely), or you can steal a victim's energy, you can use energy from ley lines, you can use energy from the cosmos, or from the sun, or from the moon, or borrow energy from dark creatures, etc.
Adjectives like "white" and "dark" characterise the source of energy powering the magic spells.