Here ya go ... a dirty ass rally car off in the woods.
This is 2006 Subaru Impreza WRX STI in WRC rally trim with night stage lighting equipment.
The paint is a blend material consisting of a complex metallic blue with another material for the graphic vinyls with the separation being controlled by a third black and white mask. I rendered it against a photo backplate and a matching HDRI for lighting and reflections. Like you'd expect, a matte material on a ground plane was used to incorporate the model into the scene. Lighting is all Image Based with a single Vray dome light mapped with the HDRI.
I rendered separate passes of just the car geometry with an ambient occlusion material and another with a Vray dirt mapped material for the dust. In Photoshop, I composited the clean render against the back plane adjusting gamma and exposures to match each other well.
Then I went to work on the "dust" layers. Using my dusty vray dirt render (solid dirt), I used various brushes to erase the dirt where I wanted the car to be cleaner. It's easier to use this subtractive method than to add dirt by painting it. Since I used a render, the dirt has light and shadow to it and doesn't look like grey airbrushed paint in Photoshop. I added another layer or two to add darker bits of dirt and grime, then dropped my AO render pass on top of it all and set it to multiply and lowered the opacity a bit.