I guess the cold elves would be those who didn't just switch to worshiping Esus, but switched to Mulcarn. There were a few elves frozen in Mulcarn's palace on the Mountain Mulyr along with the men and giants. This may not seem to likely because of elves weakness to the cold and their closeness to Muclarn's enemy Sucellus, but elven longevity does lend itself to the sloth and nostalgia representative of the ice sphere. It seems like these would have just joined the Ilians though, imho.
If I were adding another Elven civ though I'd probably start with the Once-Elves. I imagine that Haerlond is still looking for vengeance against Varn, Auric, and perhaps the Sidar that now have the Books of Laroth.