If you look at the close-up sole shot at http://forum.city-feet.com/index.php/showtopic.1786 you will see that her soles have worn, whitish areas where the skin has abraded, leaving the skin ridges prominent. This proves that this girl has had a long barefoot walk outdoors before the photo. My soles look the same if I do that. This cannot be faked! If the "dirty sole" shots you look at on the fetish sites are overall brown, gray or black, the photographer has just had the model walk around on a dirty floor for five minutes or so beforehand. Fake and boring. The girl has a nice smile, too, albeit fuzzy in the background! Good to know that some Italians are getting into barefooting, as it used to be well below their dignity.
It depends on the type of dirt too; city/asphalt dirt will do that, but plain ol' earth gives a more even coloring. I notice quite a bit of difference of how my soles look depending on where I've been walking, and I am a full-timer (literally never wear shoes, the last time I wore flipflops for a few minutes was years ago, no soles however thin for even a minute since).