nazadsam, painting Soviet trucks and Katyushas are pretty simple.
I have some of these kits as well. I also have a couple of books on painting Soviet vesicles.
When it comes to trucks, there is not a lot going on. If you don't have any reference books, using the model's box (front art and back painting guide) is a good start, but also have a quick google search should give you lots of ideas.
Overall, there were not a lot of markings, and in some cases you can just paint the truck Soviet Green and use no markings (decals).