Well there are programs like digital paint booth, but that looks aweful pricey from what I can see written about it, for what it is, specially if you only need for one truck or a handful of vehicles.
I just use a graphics program that came with my computer, and I can change color on photographs of the vehicle,use layers, create graphics ect. Most professionals use photoshop, illustrator and programs like that, but they are too pricey for me with the need I have for them. Also have an old outdated computer. I read someone mention a free program called gimp, thats suppose to be pretty good, but since I still have an old computer, I never downloaded it, cause it has heaftier requirements then my machine has. You should be able to find some decent free graphics programs out there.
Here is a drawing and pic modification I did recently. I did for fun and to help get my mind off some tragic events happening at the time, of a guy on another boards project fairlane race car, to kind of match the scheme on his mustang racecar. Shh, I borrowed a pic of a similar car off the net to modify for one of them. There are so many much more talented /knowledgeable then me working with graphics programs.
By far the coolest would be to create a model in 3d and different wheel tire packages, ect, and then nake a texture map you can paint with your schemes, and apply them to the model, and view in 3d and render the images. Much like my old nascar program, you could create your own paint scheme for the car you raced by painting a template. The milwaukes best, unleash the beast car rocked, lol.
I've wanted to make a bunch of models to do this, but ain't very good at it. Rhino 3d is a free program you can use to model (but its limited to 25 saves unless you buy it). Its the only one I can sort of understand how to operate and model in, and runs fairly well on my outdated computer. Rhino doesn't create texture maps, but you can save and different formats, and lithunwrap is a free program that can create the map to paint, and can apply that map back in rhino.
You can find a few free 3d models in a search, but usually is the more popular, sports cars you can find. I doubt you would find one of a 81 ford truck.
I've been wanting to do a few cars, but I ain't very good at it. I don't know why I can't get the hood to paint, but you get the idea. This is a 3d nova model I found on the web.
Here is a quick paint of the texture map.
Here is it applied to the model back in the rhino program. I also added some wheels and tires I had made in 3d in rhino.
