In the original Elder Scrolls, all Argonians were gray-skinned humans with green eyes. The Khajiit's appearance in that game were explained by the Lunar Lattice. But I don't know about any such explanation for the Argonians. I've heard of a race called the Kothringi from Black Marsh who have been said to have silver skin, but they haven't been seen on Tamriel since the Second Era.

The Argonians are said to become more human-looking with each lick they take from the Hist sap.

Right, hence the ones we've seen in Black Marsh look like grey skinned humans and the slaves in Morrowind looked the least human of all. Unlike the khajiiti variation and Cyrodiil's climate, I don't think these changes are directly commented upon, even if they are shown, so personally, I wouldn't put any more stock in it than I would the landscapes in Arena. For those who don't remember, in Black Marsh that was temperate/continental woodland in the north and far west and desert (with quite a lot of rain and thunder storms) in the south.

Well now I think about it, that's not much of a fun contribution. Pardon me and carry on.

Mmh. The Hist have the power to shape the Argonians. I don't know how quick this change is supposed to be, or if it's still even supposed to continue. They started out as some kind of sea reptiles, with simplistic minds. Every day the same day, hunting, hiding from danger, the world consisting only of light, darkness, nothing beyond that. From simple animal minds into intelligent agents of the Hist. Argonians still like to simplify their minds, which they can do by touching the Hist.

I can't bother to search for exact quotes, but that's all from the novel Lord of Souls, and thus I think considered canon. It's nice to think that their evolution is still continuing, thus the minor/major changes between the games.

I can't help but think that the Hist (and/or Um-Hist) are the "thing" that have achieved Amaranth. Has it been confirmed they aren't? After all, they do hallucinate and create new on their own. I get lost when MK's numbers kick in, though.