hey Steve
thanks for posting that link :) truly stunning series of pics
as can be seen the lightning isn't a different colour just because its assoc with a volcano.
those strokes out in front of the ashcloud are still blue/white, just like in a thunderstorm.
only those partially obscurred by the ash are dulled in colour to reds etc
the same thing happens in storms when the lightning is partially hidden by heavy rain bands or cloud
the only difference between lightning production in storms and volcanoes is...
rain droplets carrying charges upwards, compared to ash particles.
cheers
Dave