The blast effect texture used for plasma gun explosions can be found in models/weaphits/plasmaboom.jpg
in the baseq3
folder. Most of the weapon explosion / impact effects can be found in this folder. In this case, the plasma explosion effect is in this folder because it's a texture mapped to an actual 3D model (it's roughly dome-shaped, and it will have the orientation of the vector normal of whatever surface it hits), whereas the actual bolt is a 2D sprite with a fixed orientation (it always faces the player), which is why it's in the sprites folder.
I actually did pretty much the same thing for my mod, except I just recolored the plasma textures to a magenta color in GIMP. Hope this helps! :)