Vallejo has a line of ‘environment’ effects’ products that is great for adding weathering effects easily. As of this writing, there are almost two dozen different effects. My personal favorite so far is the “MUD AND GRASS” one show above, in one step you can can mud and organic material. You can also see the brown and dark mud effects used and blended together.
The only thing you have remember is to shut the caps tightly – if you don’t these products will harder completely in the bottle – ask me how I know. I also like to use the fuel stains to add fresh oil/fuel spills. Now, you can certainly create these effects yourselves, but these are just so easy to use. One bottle will actually go quite a long way, unless you decide to actually cover the whole base with them. I use celluclay as the base, and add these on to the vechicle and ground to bind them together. Keep in mind that these are enamel based products, so I use an old stiff brush to apply them and orderless turpentine to to clean up.
The following is a sample of some other effect being offered, head over to their website for a current list. https://acrylicosvallejo.com/en/categoria/hobby/weathering-effects-en/