[dataweaver]

Agreed. I'm fine with a spectrum-based model; and my inclination would be for polychromatic colors (mostly browns and purples; but at the extreme, white) represents the generalists of the magic community: mages who have mastered more than one color and are now exploring how to combine them in interesting ways. Calling someone a White Mage would be a sign of respect, as you're basically saying that they've mastered all seven colors, an awesome achievement.

Conversely, black could be the “color” of anti-magic; or it might just be a disparaging term for the sorry of trickery and deceit that you normally get from the likes of stage magicians: people who don't actually have any magic but are good at faking it.

In short, black vs. white might not be about what kind of magic you can do so much as how much magic you can do: more “dim” and “bright” than “aggressive/defensive” or whatever.