If you want to remain strictly canonical, I'd go with Lightning and Plant Life/Jungle/The Green. Eltarin's color-scheme, especially with the yellow cape, screams Lightning to me, while Vhisim's white, being the unique whitish-blue color from Thok, can reasonably pass as a variation on Plant Life's standard blue-green color-scheme. If you want to go with non-canonical Elements, I think almost anything could work. It'd be interesting, perhaps, to assign Vhisim an Element based upon one of his colors, and then explain the others as armor upgrades or patches, since he's got a lot going on.
As for the MOCs themselves, they are very nice, as always. The arm construction is quite ingenious, especially given its simplicity, and it makes these two very well-proportioned. I like the way the swords on their torsos function, but it does strike me as slightly odd-looking. They seem to be necessary for the head attachment, however, so I don't think removing them is necessary, especially since they do look quite a bit better in more the dynamic poses.
Nice work, as always!