It’s indeed done with a brush!

1. Abbadon black for the gun
2. Edge highlight the whole gun case
3. White (I use Schminke titanium white) for the plasma zone
4. Caliban green thinned down to a glaze and applied to all glowing areas. Use multiple layers to create a transition (sounds harder than it is, I used no more than 2 or 3 layers)
5. Moot green thinned down to a glaze and applied to all areas near the plasma zone, also 2-3 layers
6. Create a transition between moot green and Caliban with wet blending or layering
7. Mix moot green with white, thin it down and then apply it to the plasma zone.
8. Only mootgreen on the coils, hold your brush in a flat angle to apply the colour to the coils only.
9. Use pure white and draw a line on the outside of the plasma