Ah, yes, back on topic.
For reference, here's Harry's description of MacFinn (Fool Moon, Ch. 15):
I studied the man a little more closely. He was big. He was really big, at least as tall as me and twice as broad. He was dressed only in a pair of cutoff blue jeans, and those looked like they were ill fit. He was in a condition best described as "overwhelmingly masculine," hairy-chested and muscled like a professional wrestler. There was grey in his hair and beard, and there were lines on his face, putting his age at well into maturity. It was his eyes that showed me the most about him. They burned green, wild and haunted, fastened on the distant sky now, but heavy with the weight of too much terrible knowledge. It couldn't have been easy to live with a curse like his.
As far as hair color goes, all it says is "grey in his hair and beard." From the phrasing, I'd say some base color streaked or speckled with grey, rather than full grey. Logically, he's not likely to be blond, since Harry wouldn't likely notice a contrast between grey and blond at night in the woods, even with a full moon. Red is a possibility, but not indicated either way.
I checked the pelt coloration of the loup-garou, later in the book (Fool Moon, Ch. 17):
Its pelt was shaggy, jet-black and matte, except where fresh blood was making it glisten.
You might take from that quote that MacFinn's base hair color was black, though the fact that the loup-garou's pelt was not streaked with grey leaves the question open enough to be debatable, in my opinion.
As far as the curse mechanics go, my guess would be that the curse travels when the current curse-ee dies, but lies dormant if the new recipient is not above a certain age. I don't really have textual evidence to back that up, beyond MacFinn's Vietnam experience (which, now that I look it up, is more ambivalent on a point than I'd thought). From Fool Moon, Ch. 15 (emphasis mine):
He nodded. "It's how I came back from 'Nam. Everyone else in my platoon died but me. I knew the full moon was coming. And I knew that I hated them, hated the soldiers who had killed my friends. When I changed, I started killing until there wasn't anyone left alive within maybe two miles."
I thought I'd read the assumption that MacFinn's change in Vietnam was his first change. Based on the text, I'm not sure. The sentence I put in bold seems to state flat-out that MacFinn at least knew the change was coming, whether or not it was the first, which is a difficult point. If MacFinn had any idea what the curse would do to him, I think joining the military was a ludicrously poor decision. Basic training lasts longer than a month, I believe--it's not like you can guarantee your own schedule to have monthly 3-day vacations like clockwork.
If the MacFinn/Tera West theory is true, I seriously hope one of the good guys figures it out before the poor kid's first change.