There are two different things you have to consider when discribing the magic in the Dresden Files: energy and symbols.
now the energy is simply what fuels the spell, so your effect can actually take place. "White" or "life" magic takes it's power from what best represents life: emotions. "Black" or "death" magic takes its power from everything representing death, the force of ghosts for example.
There is a passage in White Night, that explains the whole energy part very good, I think:
White Night Spoiler (Hardcover, Chapter 20, Page 169):
All magic obeys certain principles, and many of them apply across the whole spectrum of reality, scientific, arcane or otherwise. As far as casting spells is concerned, the most important is the principle of conversation of energy. Energy cannot simply be created. If one wants a twenty-story column of fire hot enough to vaporize ten-gauge steel, the energy of all that fire has to come from somewhere. Most of my spells usw my own personal energy, what is most simply described as sheer force of will. Energy for such things can also come from other sources outside of the wizard's personal power.
Whis spell, for example, had been drawn from the heat energy absorbed by the waters of Lake Michigan.
So white and black magic is only the wizard's personal power, both can also use their surroundings to fuel their spells.
The symbols on the other hand are a way to shield the wizards brain from the energy he is channeling to create the effect of his spell. This is where the elements like fire or earth or the color coding thing come in, and I suppose a wizard needs to categorize the use of his magic somehow in order to prevent his head from exploding.
Evocation in the books is mostly symbolicaly divided into the elements, and I think that is, because they can all be connected to some kind of energy (fire=heat, wind= kinetic, earth=magnetic, water=entropy), which is exacly what you want in evocations, and it is for example easier to think of a ball of fire than on acceleration of molecules to create heat. But evocation can also work with other symbols, like the shield-bracelet for example.
Thaumaturgy also uses symbols to help the wizard focus his magic on the effect or object or person or whatnot he wishes, but thaumaturgy uses more time than evocations, allowing the wizard to use more symbols to create a greater and/or more complex effect than evocation. Like when Harry conjures the Erlking, he uses a lot of things to get it right.
So a spell only really needs energy, the symbols are just a way to help the wizard to not go insane by the use of his magic by shielding his brain from the raw force of his spell.
That should pretty much sum up how I think the whole Dresden magic works.
If I missed anything or made a mistake, feel free to correct me.