There are very few, but I can get them. Maybe a single page. Mostly it is him wearing the same uniform that his daughter wears. I think the last time we saw him was in "Agent of the Empire" by Dark Horse in 2013.
I have a guy in my garrison that does him. I'll put him in contact with this.
The CRL doesn't state a specific requirement. I would look in the comics, as a primary source, and do your best to emulate it how it looks best to your eye, and looks the most natural.
I didn't have a lot of luck on the internet, searching for pictures that show the back of the head... you may have more luck in the comics. I did find these two, which seem to make sense but certainly are not authoritative in any way... especially since both seem to taper the white off in different ways.
@buckrogersbarker You often have good comic references... any thoughts here?