But, the way the White Court feeding is described in the book, is that they mix their life force with the victim, transform some of the victims lifeforce into their own, then draw it back into themselves. Magic uses the humans lifeforce, so I'm thinking its probably the same pool of lifeforce, and the more you spend on magic, the less there is to feed the demon.
Biancas magic would be different, since Undead magic comes from a different powersource, and the reds and blacks feed off blood, not life energy.