How do i paint stained and filthy clothes?

[Erikjust]

http://www.beastsofwar.com/wp-content/uploads/2014/03/The-Bayou-Boss.jpg

I am in the midst of painting the big guy in the background holding the pig and the way i see it, his t-shirt is really dirty with stains on it and it probably haven´t been washed in a while.
How do i paint clothes to look dirty and stained.
I doubt it´s as easy as just giving it a black wash or glaze and dap some green color on it?

 

[Foxtail]

In that picture it looks like he's got a fairly clean shirt aside from a couple of distinct yellow stains. For these I would try painting the shirt clean then either lightly stippling on some beiges and yellows, or really thin down some yellow paint and pool it on so it forms a thicker skin around the edge.

 

[Ghool]

Easiest way to paint a stain is to use watery paint. A little thinner than a wash.
Put a dollop of paint where you want the stain.
Let it dry a few seconds - this is to create a dried ring.
Use a second dry brush to soak up the rest of the liquid.
This creates a ring of your chosen colour that looks just like any stain would.