I find nutrition is key to keeping color. I supplement zinc/copper because our hay and soil are a bit iron rich, and that can contribute to sun bleaching and skin issues.
I haven’t had any luck personally with the color enhancing shampoos (except for Quicksilver, which I do find essential for grays and horses with chrome). I don’t use too much shampoo on a daily basis, because that can be very drying and stripping. You want to preserve the natural oils in the coat. When I bathe with soap, I use either Dawn or Cowboy Magic for most of my horses. If they have sensitive skin, I have a prescription shampoo from the vet that I use instead.
I also regularly condition with the pink concentrated spray on their coats. I believe it’s the Healthy Hair products conditioner. It gives a nice shine without drying the skin (unlike things like show sheen).