In case you missed the Diablo III: Necromancer Q&A (May 3), there was a question that prompted Lead Technical Artist Julian Love to reveal there are plans to release some kind of story-driven project that will explain why the Necromancer has white hair. It is unknown if this will be a new novel, or a digital comic (similar to the Overwatch and StarCraft ones), or a graphic novel (assuming by Dark Horse), or a Short Story.
Julian Love opened the possibility for any of these to be the in-development project to tell us more about the Necromancer's history. Read the full Diablo III: Necromancer Q&A transcript (or watch the video). This quote can be found in page 4 of the Q&A.
[bluepost]Why is their hair white?
Julian: (whispers: "artists!" laughs) Well, it turns out this is a super hardcore legitimate reason for this, that is explained by lore; and I am not going to answer it today, because that is going to be answered in the future by some other media that we are going to release. So if I tell you today, it would be a huge spoiler. So you will have to wait for that one.