why is there a white streak in my hair

I have a question...

Why is it that whenever some characters go through a traumatizing, plot-driven event, their hair now suddenly possesses a white streak?

I mean…

Jason Todd

Danny Fenton/Phantom

Takashi “Shiro” Shirogane

Am I missing something? Is this a basic trait for characters who go through PTSD? Are there any other characters who I missed?