For her tail, use electrical tape. It sticks to the tail, but it's easy to take out. Just tape it up and you should be fine.
As for her color I have a Cremello horse that was super dirty and I got him spotless.
Take an old rag and soak it in peroxide and rub it on stains like poop stains and grass stains, it gets them out pretty well. You may need to use some muscle. Vinegar works too but I don't like the smell. I also usually hose my horse down after using peroxide.
I then use a whitening shampoo designed for horses. I put some in a bucket then dilute it with water. I then sponge it on my horses, let it sit for a little bit then rinse it off. Usually I do this 4 times.