another thought that you might want to consider for the argument with Dumbledore, the intention and emotion needed by some spells to work could classify dark. Those that need dark emotions like hate, a desire for pain, total domination, and rage would be dark. In cannon the unforgivable are unforgivable because of the circumstances necessary for their proper operation, loving and desire for pain, domination, hate, as well as their effects. Then spells, like the patronus that requires happiness, are light and spells that require neither could fall under your arguments.