You may not know this, but dragons occasionally shed all of their outter scales and regrow a new hide. Usually they are hibernating on great hoards. Naturally, this makes them more vulnerable to attacks by adventurers, and more than a few dragons have been defeated during thier periodic shedding. This explains why this shedding is of an unknown incriment of time.
Mymuch-beloved dragon incarnation of my great-grandad, Walt Abplanalp, has begun his once-a-incriment-of-time shedding and has turned gesso-white all over his body.
I'm not sure how long it's going to take for him to grow all his scales back, but I'll be sure to post pictures of his new hide.