I use pigments mixed with water and a small drop of washing up liquid. I just slap this all over, let dry and remove excess.

Pigments work best, but powers can work also. In this case I would recommend Youtube for some ideas and application methods. Its fun to do and certainly makes the build better.

BK

Pigments and powders are probably best. But I've had no luck with them. I picked up from Steve Zaloga adding a little 'stucco' paint (you can get at an art store) to the lower suspension. Then airbrushing Tamiya Deck Tan all over the lower sections of the vehicle and feathering it out as you go up. Then add a little Tamiya Dark Earth to the lowest sections of the vehicle. The Deck Tan looks like dried out mud and the Dark Earth like fresh wet mud. The 'stucco' gives it texture. 

Recently I found that if you add a couple of coats of hairspray to the model before the mud coloured paint you can later stroke a moist wide brush down the model. It will cause some of the paint to come off giving a rain streaked appearence. It works with Tamiya paint in any case. Not sure it would with enamel or any other non-acrylic. 

All good methods listed above. You will have to experiment and see what works for you. I like using pigments mixed in rubbing alcohol cause it dries super fast and I can go back and remove the excess and even re use the pgments when depicting light weathering. Elmers glue works well too mixed with fine dirt and RR ground cover which looks good on the tracks for a heavier effect.  If using alcohol make sure the paint has cured as it may react with some uncured brands oterwise water and a couple of drops of Dawn will do fine.

These two were done with Alcohol and MMP Powders over cured enamel and excess removed with a brush once dry.

That's excellent, many thanks.