Hi,
for that effect, you can use several technics.
THe "basic" technics :
the washes (dirt) : dirt is wet, so you can use a "wet" technic.
the drybrushes (grass) : grass is rubbed, so you can use a "rubbing" technic. you can use it for dry dirt or dust too.
the washes : you can use some ink (Citadel brand, or Vallejo), or take some paint (brown for dirt for example) and dilute it heavily in water. Then, simply put this "washe" on the figure in the places you want dirty (pants, for example). Since the washe is transparent, once it will be dried, you can add another coat if needed.
The drybrush : simply put a old brush in acrylic paint (green, brown), and remove all the excess on a piece of newpaper. The brush should be almost dry. You just want the pigments, not the paint medium.
Once the dry is almost dry, brush your figure with it. You'll see the pigments will cover the edges of the figure details.
By the way, that technics can be use for shading or highltning too.
the last "advanced" technic for "weather" effects is the use of pure pigments. It 's sold under the form of powder. You'll find it in modelling (railroad for example) shops.
You can also buy
dry artistic pastel chalk , and make powder with it. Basically, it's the same thing.
Then you take this powder on a very soft brush, and simply blush it (like women make up) on the figure. When it's done, you'll have to fix it with a spray can mat varnish (or glossy if you prefer glossy, personnaly I prefer the mat effect on clothes and skin).
You can also mix the pigment powder with some zippo lighter fluid, then brush the fig with the mixture. Once the zippo fluid will be gone, the pigments will still on the figure.
You can also mix paint or pigments with heavy gel arts medium, to make heavy dirt, or even snow...you can make snow with light bulbs, by the way.
Well....I guess I have to add an article on "my MFCA tutorial to do list" : "how to make realistic weather effects"
Soon...