You can't apply paint schemes to just any vehicle. The .dff model (3d) has to be specifically set up to allow skins to be applied. If they don't have the allowance for skins your only recourse is to try and mod the textures applied to the vehicle and this will almost always be NOT what you're after. Only a handful of the default vehicles in the game have this feature.
Search any GTA site's downloads for skins to the vehicles that allow skinning. For example, you can find them for any vehicle that can enter Wheel Arch Angels (Elegy, Sultan, etc.). Other downloads of this sort will retexture some vehicles. Examples include the Hunter, Monster truck and Rhino and many others. Instructions for installing them is usually included in the archive.
Im trying to get paint schemes like this http://www.youtube.com/watch?v=NVWesRjHwEM