Welcome! Be forewarned, customizing is like the Dark Side of the Force: "Once you start down the path, forever will it dominate your destiny!"
The easiest way to improve the Twins will be a dry-brush of gray, silver and rust. You'll need are a couple soft brushes, gray, silver and rust hobby paint, and a clear coat to seal everything. I prefer Testors for the paint and clear, but go with whatever you can get. If you have a Michael's, Pat Catan's or a model/hobby store in the area they'll have everything you need.
Here's a pic from the movie for reference:

Apply the stickers first, that way they'll blend in better.
We'll start with the rust, it's the most prevalent color on the truck. Use a larger brush for this one. Basically you dip the brush into the paint, then use a paper towel to
almost dry the brush. Start from the top, and lightly move the brush down. You should get a overall rusty effect, with the corners picking up a little more paint than the flats. As you can see in the picture, there's more rust in the running boards and fenders, so dab the tip of the brush to get more color here. If you get the paint on too heavy, use a q-tip with some thinner to clean it up. Once you're happy with the effect, let the rust color dry for a day, then go back with the gray paint, then the silver. Once you're done, let everything dry for one more day, then spray the clear coat and let it dry.
If you get stuck, or need anything, we're always here.
Don't forget to post pictures when you're done!
