First of all, awesome work. Seriously.
Secondly, on the subject of color, I'd say you should consider the effects of the environment on the statue. The predominant feature of the Argonath is the river they run over. Perhaps the stone at the legs and the hems of the robes is impacted by continuous moisture? Also, since they're built mostly out of high cliff faces, perhaps there's been a bit of overgrowth where the figures meet the hillside? Keep the figure all one color, except around those areas. That might be cool.
I eagerly anticipate the second king. Woah man.