I feed my blue roan gelding paprika and flax because he tends to fade in the summer. He didn't fade at all last summer, and I can only attribute it to his diet. I give a tablespoon a day of paprika, NOT the hot paprika, and 1/2cup of freshly ground flax a day. I just restarted the paprika this last week because now is when summer coat is starting to come in, and you need to start feeding it before the hair grows. Also check into mineral deficiencies, as certain minerals deficiencies can make the hair more likely to bleach out, like a lack of copper and zinc.
I buy flax in a 50lb bag from the feed store and it costs me about $45, I bought this bag last August, I'm almost positive it will last me close to August again, feeding 2 horses. You don't want to feed more than 1 cup a day though, as it can upset omega balance, and I prefer to feed freshly ground vs whole. Whole will keep almost indefinitely if its kept in a dry clean container, where as freshly ground will only last around a week, or less if its very humid and hot, in a dry clean container, as it spoils easily. So the best bet is to either grind it fresh, or buy stabilized flax products, which are inherently more expensive. There have been new studies saying that you don't need to grind it for it to be digested properly, even if you see whole flax in the manure, it apparently can still be digested through the shell, although its not that much more work to grind it and be positive they are getting its many benefits.