Do you have scraps of that fabric? If so, wash a scrap with vinegar and see what happens. Most modern dyes are chemically set. Vinegar helps set some organic dyes, but is probably ineffective on most modern fabrics.
Edit. Also, the fabric that faded did bleed. You were just lucky that the light fabric was one that does not easily absorb loose dye. Because of different manufacturing processes, different fabrics respond differently to dye bleeds.
Last edited by Prism99; 11-02-2016 at 10:22 AM.