My only caution with paparika is if any horse tends towards colic, ulcers, or a finicky stomach in general, I would not feed it.
As a fade preventative? It might work, however genetic color code plays a huge part as to whether a black horse fades to a dark bay by summer's end or not (using black as an example).
Years ago I an an inky blue/black Morab that was out on pasture all day long. She had shade if she wanted it but shade trees don't do much to filter the sun.
She was as inky blue/black by the first of Septembere as she was when she shed out in the spring.
On the other side of the coin, my black Tennessee Walker (who had the same turnout routine as the Morab), was faded to a dark bay by the first of September.
If the horse is a show horse, and if it's possible, it might be better to turn the horse out at night. If that isn't possible, perhaps only turning out during the when the sun isn't at it's peak and putting a light sheet on it might help
That all being said, I am assuming by "fade", color fading is what is meant and NOT a dulling of the coat. Dull coats indicate some sort of health issue that paparika won't help.