Flyspray bleaches coats like no other. Make sure if you have to fly spray, that you either find one that has sunscreen in it (if they’re even available) or after it is dry, follow up with a sunscreen type spray like Santa Fe.
Sweat is also a color killer, extreme – sweat seems to act like a colorstripper in some horses. Same for horses who get caught in the rain and dry in the sun.
There is not much you can do to prevent bleaching otherwise, that doesn’t impact the welfare of the horse. Night turnout is okay but often the bugs are the worst at dawn and dusk, and it often isn’t as much time out as most horses need.
A fly sheet will help protect the sun from bleaching too much, but you run the risk of the horse being too hot.
Paprika is what I think people use – however, it’s a little late for that as the summer coat has already grown in.
I do think some horses do have a naturally lighter summer coat - not from lack of sun, but just genetics - my gelding is a very dark bay once his fall coat comes in - in the summer he is a very warm mahogany color.
Like JB mentioned, it’s worth testing for copper and zinc - some horses can have sneakily low amounts without us ever knowing.