- last post: 01.01.0001 12:00 AM PDT
*Puts physicist hat on*
I'm not going to get too technical since it's been a while since I last studied this, but there's a couple of possibilities that come to mind.
The first is the one that's already been put forward - they use different gases within the different classes of weapons. this might be because the Covenant is made up of many races who share their tech, but who don't necessarily share the same developments, so they may have ended up producing weapons based on the same tech in different locaitons to different specs, with each area or even species now specialising in certain weapons (explaining why every plasma pistol is the same, and every plasma rifle is the same, but the two appear to have different colour plasmas).
The second is that the plasma used in different weapons might be being energised to different levels depending on the weapon. When photons drop back down through the energy levels of the atoms within the plasma, they they'll come off with a specific amount of energy that determines their frequency, and thus colour. If the different weapons bump the photons up to higher levels (say because the weapon is more powerful), then the photons given off by the shots will have different colours.
The third theory is that it's deliberate. If you thikn about it, you can set things up using the above two scenarios to ensure the weapons have different colours when fired. This suddenly becomes useful in a mass firefight - you can immediately identify who has what type of weapon among your allies without even needing to see them. With human weapons you can just about tell from the sounds of different rifles and suchlike, but if you can combine a visual element to this identification as well then you've made it that much easier.