When dyeing denim they use salt to force the dye onto the fabric. The dye itself actually has more of an affinity for the water than the fabric so the salt is used to cause the dye to be attracted to the cotton. The excess dye is then washed off at high temperature. In future washings, if you'll add more salt, maybe like an 1/8 cup to a wash load it help the dye remain on the fabric and eliminate fading. In addition for example if you're washing a dark polyester fabric you can add about an1/8 cup of vinegar with a load to help with fading there too. Hope this helps.
