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
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.........