IN no way will i take credit for this .. I Just made it easyer for people to find..
BTW: You need to know basic layer and fill commands in Photoshop. For questions refer to the Photoshop manual.
Important: The right base color
Before we start, it is very important to choose a picture, where the car has the right color. We want something that is as close as possible to light-gray.
The reason is that we need to preserve the light and shadows of the original picture. A dark car (like a red car) will distort the areas of shadow, making it very hard to make bright colored graphics. Similar, a white car will have too many areas of light, making it hard to create dark graphics.
Any semi-bright body color will do (because we will convert it later).
Preparing the car - step by step
Before we can start adding graphics, we need to prepare the car. Adjust the lighting and shadows, make reflections, and a layer for the graphics.
We will start out with this:
First select the body of the car - without the windows, headlamps, decals, etc. With the car selected create two new layers with the content of the selection. But, disable their visibility.
Tip: for convenience call these "Car", "Graphics", "Reflections"
Now we got our foundation and the only left is adjustments. We need make 2 major adjustments. The first one being that we make the car white, the second that we create a "reflections" layer.
To make the car white re-select the body of the car and adjust the brightness levels of the original layer. With white do not mean pure-white, but that the car appears to be painted white.
The power of light
Before we make the reflections layer, I want to show the importance of light - in this case the reflection layer. Many people do what we have done so far, and then just added colors or graphics using the "multiply" blending mode.
The result is this. The colors look uninteresting - there car does not seem to shine.
What is missing is light (or reflections of light) - the most important element when creating realism.
To add the reflections turn on the last layer. First change the blending mode to "Screen".
Then open "levels" and adjust the darkness of the layer until lightness of the body paint no longer shows. And voila we got realism.
The last step is the graphics, which can be practically anything. Just change the middle layer (the one called "graphics" to whatever you want - or attach additional layers to it.
Tip: Enable "Lock transparent pixels" to lock the layer.
Tip: If you want, add two "lens flares" to turn on the headlights.