There's an article in one of the past Model Railroaders in creating Ground Goop that I use and really like.........1 part Vermiculite.......1 part Earth Colored latex paint......1 part celluclay............1/2 part Elmers Glue.............if you make this in a sealed container..........it will stay for a year or so far that I know............(don't forget about a capful of Lysol if you seal it and put it away for awhile to prevent any mold)
Hope this helps.........
As one who has slipped and fallen in my share of cow pens, 2 thoughts:
1. If it is to be located where it rains often(east of the Mississippi), the ground cover would be a dark charcoal grey with a bare hint of green.
2. If it is in the plains or west, the ground would be a much lighter grey with a hint of tan.
In either case, the ground would be darker than the adjacent dirt. Also keep in mind that if a lot of livestock go through the pens, there will be a LOT of manure.
Hope this helps ---- John with the muddy boots in the pasture