Before you actually start painting it is good to have a notion where your light source is coming from. When you don't have an obvious light source in the sketch it is up to you to decide where you want the light to come from (Fig.04).
With the light sources defined it's time to start painting. I created a new layer under the sketch and start laying down some washed colors to give myself an idea of what color scheme to look for (Fig.05). I kept the light sources in mind as I moved around the image, painting in both the light and shadows.
Normally it's better to start with the background, especially if it has complex light sources and details, because the overall look of the image will depend on it. But in this image the character, besides occupying a large portion of the illustration, has the main light source beneath him so I started with the character and the machine in the first plane (Fig.06).
At this point I just used some basic colors for the highlights. I already had in my mind that the energy light coming from the machine would be a very light washed cyan. I put some in the character and also in the areas that were going to be lit with the energy. I also started working on the different pieces and colors of the machine.
I always paint directly into the same layer. I think it gives more of a feeling of a real painting and the colors also blend with each other much better. Starting with the shadows and using a custom brush (Fig.07) I started adding dark tones to the painting. The highlights from the skin gradually became a very light washed cyan as they're closer to the energy light coming from the machine. I don't know if it's noticeable, but as I went along I started cleaning up and fixing some more rough brush strokes.
As I said I always paint on the same layer, but to have some control I separate different elements into masks that can be accessed in the channels. As I move along I start adding some details here and there, and sometimes the creation process becomes a little chaotic. With this image I worked a lot in the foreground before even doing anything on the background. Normally I don't do things this way, but I was really looking to show good contrast so that when you look at it you don't get distracted by the background (Fig.08).