[QUOTE=M. O’Connor;7131906]
DYING? That’s pretty drastic…and uncalled for as well. Traditional methods, grounded in good horsemanship and nutrition will suffice, and yield great results without waiting forever.
At this point in the year, a horse is shedding his summer coat and beginning to grow a new one.
So for starters, spend time currying your horse at least daily, and maybe 2x daily, as part of his regular grooming routine.
Bathe him only as much as needed to keep him clean, and no more. Use a flyspray with sunscreen in it.
Invest in a good flysheet, one that covers him from ears to tail to keep the sun off his coat. (I know he’s leased–give it to him as a farewell gift to his owner when you’re finished with him, or keep it for your next horse, or even eBay it if you need recover some funds).
The most important point to look at is to be sure he’s getting good nutrition, including good hay, a grain fed at a rate sufficient to comprise a “complete” ration, or if he’s an easy keeper a ration balancer to ensure he has adequate intake of vitamins and minerals, and a hoof/coat supplement with maximum biotin. I have fed BOSS (black oil sunflower seeds) for years, and when we put a new horse on them (about 8 oz-ie one cup- am/pm) we can see a difference in their skin and coat within a week.
If your horse isn’t already getting a hoof/coat supplement and/or BOSS, now is a great time to start supplementing as the new coat will have the full benefit of it.
Also, go out and examine your paddock, find out where your horse is rubbing (if he’s curried regularly he might not itch so much) himself and getting scratched up. A hotwire will keep him off the fencing, saving his coat, and the fence as well! Any other sharp protrusions should be eliminated. Being diligent about this is just good horsemanship, and will keep your horse safe from injury, no matter how slight.[/QUOTE]
All good points, but just to clarify, I am definitely not considering dyeing him. (no offense to anyone who does dye.)
My original post was just about what to do to clean him up as best I can prior to a last minute event we have coming up in a few weeks (e.g. whether clipping him would help to even him out, thus masking the unsightly black lines a bit, which is what I’m really concerned about.)
I’m not too concerned about the sun bleaching. We’re in Arizona. To an extent it’s unavoidable. He’s on supplements, UV protectant fly sprays, etc., only bathed when necessary. I curry the living daylights out of him twice daily (I’ve always been obsessive about currying. I was raised in Pony Club back east with my own mom for a DC and she’s even more obsessive than I am. Pony club is all about the currying.) There is nothing unusual in his paddock. I check regularly. He’s probably just scratching himself up on the hard Phoenix ground. I’m used to the soft, rolling green hills of PA/NC where I grew up. This desert stuff is new to me. 
Thanks again!