I am using the 18-55 lens that comes with the xt. About half of them are at ISO 400 and the other at ISO 200, at 18mm, F4.5-F8. And I guess I spoke too soon about not overexposing as I just had some wonderfull lightning that was too close and too powerfull and it overexposed badly =(
I think the color change has something to do with distance and/or the strength of the current at that particular point...ie weaker/farther= more purple as compared to the main channel or other lightning in the shot? could be wrong but thats my 2 cents.
BTW...Doug that lightning shot is crazy awesome