Ah, I was hoping you would ask that...let me explain...
theory 2: The DB and White Hair...
When Nero was thirteen years old, the order of the sword, expirmented on him. They wanted to see if they could take the blood of Sparda and inject into a single human extermity...they injected Nero's right arm with the blood of Sparda (well techincally it was Vergil's blood)...Nero's hair turned white...but since nothing immediaetly happened to his arm...the order of the sword considered the expirement a failure....