I mentioned this in the Discord a while back, but figured this is an appropriate moment for this - if the voltage required for the Ray Gun is between 220-230V and uses two (Cold Cell) Batteries in series (above picture is how they are put into the Ray Gun when reloading), that means each battery has about 115V. Cheeky Treyarch.
Since plasma is superheated gas, it's not too far of a reach to say the high voltage overheats the gas as it leaves the cell, creating the plasma discharge. Plasma coloration is based on its ionized energy level; that is, other elements involved in the plasma will change its coloration. Given that and the color change after Pack-a-Punching it, I'd think that green is the energy color for the basic 220/230V Ray Gun and the red is when even more aethereal energy gets mixed in with the plasma, vastly increasing its ionized energy (and damage).