k, i made them go in straight lines, not sure why i did random anyway... each firework is now a totally random color, and just looks better overall. http://www.box.net/shared/n1eqvhxyin
hmm... think the particles after the fire work has exploded should not be just coloured dust, i reckon that they should be weightless, and that the "spawnable" material should burn for a random amount of time(no more than 5 second), then at the same time have a random chance of igniting an adjacent particle/ (not all) than shoot up and explode.
I have no idea if that is possible for you (i certainty could not do it), but its a very cool idea.