In Photoshop there are a lot of ways to reach a determined result. Is up to each user define and follows certain techniques. I [have done] two Photoshop classes (intermediate and advanced) years ago, but now I certainly know much more than I learned in the past. I [have used] Photoshop [for] about 11 years and always try to apply the tools for illustration purposes. To reach this results in precision use the Photoshop selection tools. In fact, there are other tips to get this, and this [tutorial] is a result of years of attempts, success and failures with Photoshop).

Each image posted has its own layer as follows:

-1 layer for background
-1 layer for the line art
-1 layer for the flat colors
-1 layer for shading
-1 layer for second level of shading
-1 layer for the lights
-1 layer for the highlights (maybe I don´t use this)
-1 layer for contour (maybe I don´t use this)
-1 layer for effects (glowing eyes, and so on)

At the end, the layers will be merged and we get the final result.


Basics: Line Art and Colors Layers:

1) Line art: I drew these lines in 2007 and decided it's time to add some colors on it:




2) Basic colors layer: Yep . . . exclusive colors - it´s a recolor/repaint (just playing around with Photoshop selection tools):




Shading - First Level:

Now the first level of shading. I use black with 35% of opacity:




Second Level of Shading:

[This is] to give [your work] more contrast and that cool 3D feel . . . .

In a new layer, I added more black with 35% of opacity (again) to define were the lights don't reach or reach just a bit. This helps a lot and the 3D feel will be strong at the end:





Lights:

[I use] the selection tools of Photoshop a lot here. For better results, (this is valid for any kind of selection) select the desired area with the "magic wand tool" and then at the suspended menu: Select > Modify > Expand (1 or 2 pixels, depending on the situation) - then use the Gradient tool with white as foreground color and pick "foreground color to transparent" in the Gradient tool options. Apply the gradient according with the light source direction (from right to left, above to below on this particular case). The intensity of the gradient will depend on the light source. You should be careful with this for better results. (Note that the top of the Alternator is brighter than his feet). Try to visualize the light source and be coherent with it:



I use a darker background to show the light layer properly.

***VERY IMPORTANT NOTE***
The light complements the first layer of shading but not WITH the shading layer. The select area for the light should not contain any shaded area.


Highlights Layer:

When the lights pop up from the eyes (the main source light reflection, or whatever), use the selection tools again. You can also use the pen tool to generate extreme precision select areas. I did in determined areas. After [this], apply the highlights with solid white. You can add some outer glow layer effect. Shinny!




And finally . . . .

The Effects Layer:

Here I concentrate my firepower in the eyes, rear and front lights.




MERGE all showed layers and get the final result.

Finished:




Another important note about work[ing] this way is the fact [that it] is easy [to] do an instant repaint. All you need to do is change the flat colors layer. The rest is already done!

For example, take a look on this "Burning Alternator". I prepared this repaint in 2 minutes:



I hope you like it! Bye!