WELL changing the color, or adding decals and changing textures involves going into the car/truck's .zip file and editing the png/dds images in it, with a tool like Paint.net...
Experiment as much as you want, Just keep a backup of the file just in case you screw something up.
As for physical parts like rims and kits, You'll either need to design the parts yourself in Blender3D and add a few lines of code so that the game takes it as an external part, Or check out the Parts subforum for some vehicle parts other people have made, and read about how to install those parts. I don't know much about it, Check out the game documentation for more info.
And to mod the vehicle's speed and performance, You need to edit the car's ".truck" file, in the car's zip. Don't edit it randomly, It has syntax. Refer to the Documentation.