HP: Why Is the Transfer Student's Disarming Spell Green?

Lin Du, a god-forming ancestor who traveled to the world of immortality for three hundred years, was almost knocked to pieces by the World-Destroying Divine Thunder. When he opened his eyes, his body returned to the age of 11, and he was "assigned" to Hogwarts as an exchange student by the cheap masters. Faced with the little wizard who only knew how to "expel your weapons" and the Dark Lord who didn't even dare to mention his name, Lin Du held the violent black dragon wand and fell into deep thought: Is it still too late to drop out of school and return to the sect? Until one day, the Dark Lord finally returns and the whole school falls into panic. In the midst of the chaos, Lin Du calmly raised his wand, and a strange green light flashed—— Voldemort looked at him, and he looked at Voldemort. Lin Du: "Sorry, it's an occupational disease. By the way, do you still want your Horcrux?"