So Jason's white streak only actually appears, in the Main Continuity (Elseworlds are a different beast), in Hush. As we all know by now, that was promptly stated to be Clayface.
HOWEVER.
Winick said fuck that, it was so Jason at the time and you can't stop me from making that canon, and sure enough, when this is explained in Under the Hood and we see him noping off to his hidey-hole, he has the white streak. He does not, however, have it during the main storyline. (Maybe it wasn't working with his Robin Bangs, I dunno.)
Two possible explanations stick out.
A) He did have one and it wore off. Unlikely; he doesn't have it in Lost Days at any time whatsoever, but maybe he was dyeing it. Or it came in later.
B) He dyed the white in, for the Supernatural Aesthetic, and Talia either helped so he wouldn't bleach the fuck out of his whole head OR threw up her hands and bailed because in the grand scheme of things, this is not a battle worth waging.
Bonus! If we entertain B, that explains why the helmet is on for the first several chapters of UtRH; he's in that awkward grow-out stage and it'll ruin his whole Thing if he looks Like That.
