Well it still may be possible to lay some lighter hair over the existing hair, perhaps by gluing it down with something not too strong (like a light pass from a glue stick). And then you could have the hair going in whatever direction you wanted.
It would be delicate work, but I suspect the end results would still look a lot better than from painting (which I just really don't see working too well, or not looking like ugly painted hair in the end).
This is along the lines of what I said several pages back. In theory, a fine powder or misty (almost 'dust') application from an airbrush could work; but it'd take a ridiculously light touch and practice on something else -- something cheap -- before even attempting it.
Ideally you'd separate thin layers/sections of hair you wanted to apply the colour to by means of a card or foil, masking everything else and hitting only a few strands at a time. That would circumvent clumping and over-saturation.
Adding strands of lighter coloured hairs (I might harvest them from good quality paintbrushes...horsehair/camel-hair, whatever) would be meticulous and time-consuming but
maybe the better option. I work in creative and I'm a trained goldsmith, so meticulous and tedious work isn't exactly new to me, but I'm not likely* to pick up Chewbacca anyway so I'll leave it to someone else to attempt.
The problem with that is you could find yourself altering the contours and shape, adding too much material...hard to say without a visual inspection of the actual figure.
My final thoughts on Chewie without having seen him in-hand...he's one hell of an impressive figure and iconic Star Wars. The OT wouldn't be what it is without him, but I'm just not a big fan of the character. Re-watching ESB, I actually found his braying and yowling annoying. It's kind of a love-hate thing with him, to be honest.
If Hot Toys had nailed the underlying structure of his face better...the brows, the set of the eyes...I may have been more easily swayed. But looking at in-hand photos of a properly futzed figure...he's pretty damn great. The pros outweigh the cons by a fair margin.
*(I say "not likely" because you never know.
...but if I was sold on him I would have bought one by now already.)