Hi Maddy ~
Rusty looking blacks are often a sign of copper deficiency. Even if your feed and supplements provide an adequate amount of copper (90 to 125 mg for an “average” 1,000 lb horse, high iron/manganese or sulfur levels can interfere with absorption and cause a “relative” deficiency.
Most color enhancing supplements contain additional copper (and, sometimes, other trace minerals), plus fat and Omega-3 sources, all of which will enhance coats, in addition to paprika.
You can test your hay for excessive iron, manganese and sulfur levels and check your water company’s annual report for sulfur.
There’s a really good explanation on coat color, melanin, etc. here:
in Vail AZ