This is the method I use to paint green plasma weapons. I like to think that the plasma will be the hottest/brightest at the bottom of the coils where they meet the rest of the gun. So I like to paint the coils brighter near the bottom and darker near the top, where the coils will be the coolest.

Coils

1. Base coat the coils in Bold Titanium White
2. Apply a few thin layers of Moot Green over top of the white
3. Being careful to avoid the recesses between the coils, paint from about halfway down the coils to the top of the coils using a thin layer of Warpstone Glow
4. Paint the very top of the coils using a thing layer of Caliban Green , again being careful to avoid the recesses between the coils
5. Apply a mix of 1:1 Lahmian Medium and Flash Gitz Yellow to the very bottom of the coils. Let it pool a little and use a brush to draw it up into the coils, about halfway, in a sort of bell curve shape.
6. Repeat the previous step, this time drawing the paint about 1/4 of the way up the coils
7. Thin down a mix of 2:1 Bold Titanium White and Flash Gitz Yellow using Lahmian Medium and apply it to the very deepest recesses at the bottom of the coils, focusing towards the center

Glow

1. Apply a thin coat of Warpstone Glow around the rim surrounding the coils
2. In the corners, the center, and anywhere the light would be the brightest apply a thin coat of Moot Green
3. Repeat the previous step, this time being much more conservative in your application, using Flash Gitz Yellow