I've has a quick search and found a few bits:
One tutorial says:
On a dark background, paint a line of hawk turquoise where the lightning stroke will be. Then highlight with progressively thinner strokes using: lightning blue, ice blue, and skull white.
Here's a pretty good quick tutorial on how to paint lightning in Gimp. Not model painting I know but still related:
http://kalaalog.com/2008/11/30/painting-glowing-objects-lightning/
Plenty of source material on Google Images.
Hope this helps....