I use our Lansinoh baby wipes. They don't have alcohol in them and figured if they are gentle and moisturizing for my baby's butt then they should be all right on cow butt too :)
Siobhan, wife to my best friend, Trent, and mommy to Camden (11/28/05) and Bennett (7/11/07). and Truman (7/28/09)
If it isnt a light color, and even if it is I would use a plain glycerine soap. It will replace some of the natural oils and sheen to the leather. I wouldnt get it too wet and I would rinse and dry thoroughly.