There are four trigger actions under "Lightning - " in the trigger editor.
Lightning - Create Lightning Effect
Lightning - Destroy Lightning Effect
Lightning - Move Lightning Effect
Lightning - Set Lightning Effect Color
You obviously want the last one. The first three numbers are based on a decimal percentage and represents RGB (red, green, and blue). The last one labeled "alpha" represents the transparency of the lightning (don't worry about this one, just leave it at 1).
GL ^-^
EDIT - oh, you can't change the color of lightning from object editor spells. Unfortunately there is no way to do that. You can only change the color of custom-made lightning in the trigger editor.