This method behind this is actually very simple all you have to do is blend from dark to light in a gradient. See, easy.
Starting by painting the weapon I like gunmetal as a base-color for most of my weapon bits because its easier to highlight most materials than shade them should I want to weather or edge them.
The plasma coil is going to be blue in this case so I started from Vallejo Game Air electric blue.
The first highlight is done with Vallejo Model Color sky blue. When loading you brush make sure you dont have too much paint on the bristles in this case so as to only hit the raised edges. Keep some of the base color around the edges and in the lower areas exposed at this step.
Next step is to add some a small amount of white to the sky blue and paint in a smaller area keeping a small amount of the sky blue showing, like before but in a smaller area. The last step is to use white in the smallest area at the very hottest point illustrated in the finished plasma gun. See nice and simple.