(This will sound even sillier, but where do we know Robespierre's natural hair colour from, anyway? - I think I've just absorbed it such that I've lost track.) I was wondering that - I have no idea how it works in hair. And of course the wig thing came up too, but it just seems silly that someone would take a clipping from his wig and call it a lock of his hair. However inseparable they apparently were. And it doesn't look like powder...
...bleach?

That was all I could think of, too - but it seems very vague! As you've said, that could mean anything. Not to mention the way it's relayed - "he said that he said that he said..."; well, you know, maybe Danton didn't say that, or maybe he did but he was making crap up. Alas.
I've just bought Gérard Bonn's biography, which was published only last year. I can't go back and check at the moment because it's already packed up (I made a brief effort to find it, decided it wasn't worth it, sorry), but I have to admit, I checked the index when I got it (okay, this point has been privately gnawing at me) and saw nothing of the sort - I don't know though if he says anything about this vice of Camille's. But anyway, I'm not sure how to take the book, since upon skimming the author comes across as a bit of a squealing fanboy (I read an entire! paragraph! where every! sentence! was punctuated with an exclamation point!) - well, he does say in the preface that he's not a writer or a historian.
I have yet to lay hands on a copy of Claretie's biography. :/ Prim or not, I'd love to see it eventually.