Might I suggest Horseman's Dream Purple Power Shampoo as well? This stuff works wonders on any horses that have white. I have used it on my paint as well as my other horses socks. It is also good when you need to get your horse looking sparkly white real soon, such as before a show.
Use SHOWSHEEN I swear by this stuff!!!! I get I at TSC or rual king it's like 13 bucks it last me quite a while.... It's also a detangler.... My horse had a super thick ratty mane 5 sprays combing you were perfect!!!(doesnt whiten though but makes her shine!!