Keeping them up during the day is the only fool proof way to do it. I have a light colored buckskin so his coat isn't a problem, but his mane and tail are a challenge. I always have to dye it back to black for the show season.
We used to show a black stallion and to keep his mane and tail from turning red from sun bleaching, we didn't turn him outside until late afternoon and let him run until morning........then stalled him during the day, or kept in our covered arena.
We tried coloring his mane and tail initially and it was very harsh and didn't look natural, IMO.
Feeding BOSS (black oil sunflower seeds) helps intensify color, and there are commercial products like Black As Knight that are made to do that. My showhorses generally stay in during the summer days though - the only sure way to protect their coats.
We keep our show horses in during the day and out at night to help keep their coats from getting sunburn. you can also thin the mane on the side of the bleaching and flip it to the otherside for showing. jennifer
We use Nice N Easy hair dye on our bays and blacks manes and tails. Leaves them very silky and very natural looking. I tried the spray on color and that looked fake and felt like straw.
Since we clip with a 15 blade less than a week before the shows (depending on the horse - 3 - 7 days prior) the fading coat isn't really an issue.