Severus stares at him for a moment, mutters ecclectics to himself (with profound hypocrisy), and says, louder, "I meant about the color in the finished potion being an energy-augmenter. You never had that from me or Slughorn."
"What have you found about the anomalies," he clarifies. "I will, though, refresh your memory by saying this: nearly all ingredients have multiple qualities. The art of brewing is that of evoking specific qualities from each ingredient and binding those evoked qualities with those of other ingredients. That is the function of color, light, waves of sound, exposure to metals and woods and other ingredients, temperature, air particles, runes, arithmantic arrays, stirring patterns: to evoke the desired qualities only, of the myriad possible, and to coax them to weave together into a harmonious and coherent whole. The whole process of brewing is that of reminding pieces of nature of things they stand for and embody, and convincing them to broadcast those aspects of their essence on an object. Sometimes that object is the drinker, sometimes it's merely another ingredient. That is also why most effects can be reached by brewing through more than one recipe.
"And that, in turn, is why it doesn't matter if you don't posit in five minutes on your first try a recipe that took a me months to perfect, Quin," he adds with a half-smile. "In the mastery of brewing, the creation rather than the assembly of a potion, you have to have a tolerance for not being right first time. If it ever happens, it won't be in your first ten years in the craft, and will be preceded by weeks, at least, of research and lesser experimentation. What I'm telling you to practice now is the ability to select ingredients with qualities which can be convinced to work together, and to so convince them."
He puts the seeds into a mortar and grinds them for a few moments--once widdershins, twice deosil, three times, traces a rune over them, and speaks as his hands repeat it twice. "I think that because the Blacks are mad, not stupid. You were sharp with him. He's observed that you interact with me voluntarily. The question will reconcile those facts. You're thinking of Pettigrew, although I don't think I would have called him rabbity."
He puts the pestle down carefully; it clicks. "Quirinus, I've spent the majority of my adult life failing miserably to dissuade both my students and my peers from attacking each other under the guise of advocating caution. I am tired of it. Let me be plain. I will teach you, I will listen to your music, if you like, although not to anything involving raccoons or similar. I will not join or approve you in disturbing a peace I never expected to live to experience. You have been violated and used deplorably, and those you can rightly blame have been removed, however unpleasantly, from your ability to bring to justice; that you have rage needing an outlet is expected and to be respected. However, I would greatly prefer, myself, for you to purge it constructively than destructively, and, if you must, then not in my name."