There really isn't much you can do about that. If you can, turn out your horses without halters. I leave my horses halter on when she is turned out just because its easier to catch her.
I really just accept that face that she is going to get them dirty and then when she needs a new one for a show I'll just buy her a new one! They are only 20 or more so its not a huge cost.
Here is the color I have on Ceilidh now, she got it for Christmas. I can't wait to see what it will look like after a winter out in the pasture