Hello and thank you for joining our forum.
Timing is everything and this time it seems like your pony hasn't helped you out much preparing for the show.
I had a light grey horse that was always rolling in something (like horses do) and getting stains all over his coat.
I had a lot of luck with a product called White 'N Bright, it helped to whiten and brighten my horses coat (it smells good too

). This product should be sold in most of your local tack stores. You may have to repeat the process on those stubborn areas but overall you should see the results your looking for.
Good luck at the show.
Mrs. Clean