The great painters of the Renaissance period are best known for the way they were able to capture spiritual scenes which seemed to glow with internal light. But you don't have to be inspired by the numinous to want to capture light effects in your renders, perhaps you just need to capture how sweet that concept car looks with it's purple under glow, or you really enjoy designing lighting fixtures and want to have a good image of the completed product.
Before you get started you'll want to first have a basic understanding of light and shadow, and understand the techniques for laying down your choice of media.
I made it at TechShop.
Step 1: Anatomy of a Light Source.
1. The light source itself.
2. The 'aura' that the light source is emitting.
Practice making soft and hard transitions to create layers of luminous light source and light aura. For this simple tutorial I'm showing mostly examples of light in space, in more complex drawings you will usually have your light source interacting with other objects in the image. When your light source is casting light onto other objects, the color of the aura will reflect off the surface of the objects.
Step 2: Spirit Glow Ball Render
This one looks weird because I've drawn an orange light source emitting a green otherworldly light. When drawing light in color it's a good thing to have a color wheel on hand. You can get a very natural looking light effect by using split complimentary color schemes. On the previous step, I had a yellow light source emitting a reddish/purple aura onto a purple background.
You can also use an analogous color schemes for more illuminated areas. Try creating a still life where all the objects are rendered as if they were a mild light source.
Small update: Here's a cool flash thingy color wheel that shows color relationships.