Minerals are a biggie for color. Especially copper and zinc. If the hair had come in white, I'd tend to say that it was done and would be white forever, but then my white horse comes in black with re-growth. Yours sounds like healthy, rich new growth. I wouldn't shave it off because you ruin the defense of the area against future rain rot. Supplementing minerals will help a lot with color and eliminate summer bleaching among a lot of other things. I've seen horses with their copper needs met and literally change color...their true color coming out. Never underestimate the benefits of those traces minerals! In the long run, your horse will have a brilliant true color and those patches will be taken care of in the process.
So, I suggest a good vitamin/mineral supplement, because Vit. A is also a good deterrent for rain rot.
I had something going on with my white horse's back last winter. It wasn't rain rot, though. Something else. He's bum high and has a small depression in his back that holds water just long enough to start something. I had gotten Thrush Off, which is also copper and works great on feets. I cleaned the area on his back and puffed a little bit of powdered Thrush Off on it and it stayed there for a long time. I just kept watch to make sure the the healing continued and it did, never looked back....one cleaning, one application and through continued rain storms. Stuff's great for dry thrush medication in the face of constant wet weather as well.
Hope this helps.....