Is it lined with synthetic or real fleece? It is hard to keep anything white looking clean around horses, but your post said it doesn’t just look brown, it is still dirty. So is it stained, or are you not able to get the dirt out? Are we talking about sticky clumps of dirt, or gains of sand caught in the fleece? Is the core leather of nylon? I’m thinking leather, because the girths based on multiple layers of heavy nylon webbing can be washed in the washing machine, and that is an advantage of the non-leather girths. Is there any reason you specifically want or need a white girth? How are you washing the girth that it is staying dirty enough to rub? You may have a problem finding a fleece sleeve wide enough to slide on over the fleece already sewed onto the girth, since the stitched on fleece could resist allowing the sleeve to slide on.