I have a buckskin pinto yep got a strip of white in her mane. I separated out all the white. Then i braided the white best i could and covered it up with foil then the roots that stuck out i covered with vaseline. Also did the base line of the neck so i didnt get black where the golden body coat was. Also found this awesome product that is called OOPS! It removes hair dye with out bleaching out the coat. I always seem to find a black spot after the hair has set and even if you clip after you dye with a light colored horse sometimes the dye will get to the skin and show up as black spots when your done body clipping. The oops product is great. I do also tie the mane up with clips after i dye to try to minimize amount of oopses.
I also found that after i thought i got all the black dyed and rinsed out there were still red strands that i missed. Theres a cover up crayon that you can get that some people use for grey hairs. Just touch up on show day to keep from having to re-dye. Luckily for me her tail is white.
Also peroxide usually takes dye off your hands if that doesnt work bleach is the last resort.