Ok, I'm half decent at painting, so I'll give ya some tips.
First, I got all my skills from taking photography classes at school, then transferring what I learned from that into ROR painting. Therefore, yes, I use photoshop. I'm sure you can do most of the stuff in paint.net and Gimp if you don't wanna buy it (it's pretty expensive lol). There's just a couple easier ways to do stuff in Photoshop.
Next, start with a bake, and fine tune the base color over it. Then start from the back and go to the front or vise versa. If there's a pretty complicated character or design on the truck, open up a new image and either trace off a lower quality picture or free hand it from reference photos. Sometimes you can get lucky and find some of the designs through Google and different monster truck websites.
Once you are done with the paint, just a little suggestion i do, overlay the original bake to the finished paint so you get realistic shadows.
Just takes some time and patience to make something really high quality. Have fun!