When in doubt I go back to what to what I was taught in 4-H 50 years ago. Disolve 1 c. salt per gallon of water cool, soak fabric for 30 minutes. Washing in cold water is also best for natural fabrics especially black. Man made fabrics that are vat dyed ( while still in a liquid state befor being formed into treads seldom fade, but man-mades should