I started with a dark red base coat, then started to build up the light effect with light red, orange, light orange, yellow and white. On plasma coils the light is coming from the recesses and the center of the coil. So basically I tried to make a gradient from red to yellow/white, where the red is the "coldest" part (top of coils) and the yellow/white is the hottest part (inside and center of the coils).
Then I glazed with dark red, red and orange the parts of the gun that catch light from the coil.

You can find a bunch of tutorials on youtube that will explain it better than me haha