Got a question about the white streaks in Lucian and Matthias' hair!
In their world hair turns gray when a person touched by something spiritual (otherwordly, the “other side”). These can be spirits, ghosts, demons and much more. Lucian's hair turned gray gradually due to certain events, this happened four times - at his 6, 11, 23 and 27 years old. Matthias's hair turned gray once (you could see that in some pictures he has white streaks, and in others he doesn't).

Also here you can see how Lucian's hair changed last time (in his 27th).
https://snakes-in-mirrors.tumblr.com/post/728010527630032896/completely-broken-this-is-one-of-the-darkest