Gallop I heard works wonders they have shampoos for all colors also ketchup works for white tails :)
If you are short on cash for the main and tail - vinegar gets the tail really white. A little on the body also helps with itchiness. I go through vinegar in my barn but it's cheap and works well.
Quick Silver and Deep Purple both worked wonders on my previous trainer's old grey lesson horse!
Baby powder is really good on my horses whites, I either leave the baby powder on and just dust it off with a brush or you can turn it into a paste with water and let it soak for 5-10 m
i love my orvus shampoo! technically is for livestock but works wonders on dirt and stains! make sure you rinse it out really well though!
Head and Shoulders!! Not only whitens it is good for their skin.
E3 brightening shampoo is really good!