Indigo dye is a different kind of dye it is a natural dye. It does not really ever permanently set like other dyes. If it did set like other dyes , our jeans would not fade they way they do... much different from other dyed fabrics. Ever wonder why the knees and wrinkled area around the crotch get those faded lines... its the color coming off from friction.
A few years back I had professional movers , and they( the moving company) forbid the movers from wearing jeans , because if they rubbed up against white walls color could come off onto the walls.