I can see where you might be heading with that dirty old man look. Very intriguing. I like the way you think.
At first glance I prefer the dark hair. It seems to fit a character who might always try to look young anyway.
The gray/old would seem to require more internalization from the character... while the dark/younger... more extroverted?
I could be easily convinced either way.
Can we add a scene of Googly Goo combing hair dye through his hair and just use both?
In other words... you get no help from me in making your decision.