The first thing I did, after briefly visiting the men's room, was buy some tostadas and acrylic paint.  Tostadas are good for painting on because they are firm and have a nice, rough texture.  Real artists refer to this as "tooth."  Surreal artists call it "throat warbler-mangrove."

The colors I used were iron oxide yellow (sort of a yellow ochre color), raw sienna, burnt sienna,mars yellow, burnt umber and payne's gray.  I have found that these colors pretty closely approximate the various shades you get when you burn a tortilla.  I used two brands: Golden and Utrecht, but the names don't really matter.  Any brand of acrylic paint will work. And probably other colors would work too, as long as they are in the dark yellow-brown end of the spectrum.  But the Payne's gray is critical.  Works great for the the burned-looking bits.