I always try to get some sort of interesting lighting scheme. Sometimes I get realistic lighting, and sometimes I prefer to get what's called "stage lighting". Meaning you normally wouldn't see lighting like that in real life, but it works great for compositional purposes. For example, when you see a movie and a person's face is fully lit from all sides, yet so is the person they're talking to who's facing them. Where could that light possibly be coming from? Or to a more extreme level, when you play a video game and there's always a light in the direction that you're supposed to go. Where as in real life it might be harder to find that door to the next area. The lights are designed to lead your eye a certain way, lead your character a certain way, or set a mood. This is "stage lighting" because it's fake, believable light that serves as a compositional tool.
In the example above, I didn't necessarily have a purpose for the lighting, but it was more of a test to see if I could get some sort of mood and atmosphere with it. To achieve this I paint the character and the background in fairly flat, darker colors. Once the values are all there I use a "color dodge" layer in photoshop to paint some hot spots on the robot. I hardly use "color dodge" anymore and I don't recommend it unless it's used VERY lightly. After that I make a new layer and set it to "screen" and with a soft brush I start painting in light coming from the top right. "Screen" layers are GREAT for lighting and atmosphere. The reason is because a "screen" layer mixes the color you've chosen with the background, but also lightens the background and diffuses it as well. In real life, atmospheric perspective does the same things. Like when you see mountains that are far away, they are usually just a blue silhouette. The blue in the atmosphere mixes with the colors of the mountain and also the dark and light contrast fades away as well. After adding the lighting with the screen layer I then add a shadow layer set to "multiply". I pick a blue color for this to not only compliment the yellows and oranges, but to show the shadow is much cooler than the light. This happens in real life too, you'll notice a lot of times shadows have a blue or purple tint to them. "Multiply" layer is almost the exact opposite of a "screen" layer. It mixes colors with the layers below, but it darkens them. Great for shadows. This not only balances the composition but makes the light seem brighter and hotter than before.
Lastly, for added atmosphere, I always like to throw dust and particles into the atmosphere. Showing different levels of particles in the foreground, mid ground, and background gives the feeling of depth. Rain, snow, bubbles, dust, fire embers, all of these help make the subject matter seem like it's in a 3D plain rather than a 2D one.
So I hope that all helps! I'll do a more in-depth lighting tutorial later on so watch out for that. And last, but not least, here's the video of this painting.
