Typically, one would spend a lot of time to get a model painted and it will usually look quite beautiful. Well, that does also depend on your talent. But even so, is that how things look in real life? No. In real life things have a tendency to look used and beaten. You will often find soot, dirt, and rust on every day items.
You can accomplish this approach in a variety of ways. You can try dry bushing the needed color, heavy washing, etc. But the best approach is to use a weathering powder.
A weathering powder is a special dust that can be used in a variety of ways. You can brush it on dry if you are looking to create a dirt or soot effect. You can also set some aside, wet it down and put it on wet if you are looking to make a rust or mud look.