This is the info:
Sometimes, non-dyers will recommend that you soak your new garments in salt or vinegar to "set" the dye. Unfortunately, neither salt nor vinegar can set commercial dye; the only value to the process is as an extra washing that helps to remove some of the extra dye. Retayne is far more effective, making colors last much longer, even after many washings.
Retayne is a dye fixative that will set most commercial dyes. It contains positively-charged particles which will stick to the negative particles of most dyes, helping to fix them in place in your fabric. Dyes that would otherwise bleed a little in every washing become safe in the laundry after treatment with Retayne. Retayne is extremely valuable for quilters who must make sure that their differently colored fabrics will not bleed onto each other when a quilt is washed. It is essential after dyeing with all-purpose dye, such as Rit dye, which would otherwise tend to bleed in the laundry. (It is not necessary for use with fiber reactive dyes, such as Procion MX dyes.)
There are two downsides. One is that the indigo dye used in blue denim cannot be fixed with this sort of treatment, because indigo dye molecules lack a negative charge for the fixtive in Retayne to stick to. The other problem is that cationic dye fixatives tend to increase the fading that is caused by bright light. Clothing that has been treated with Retayne should probably be dried indoors, not in direct sunlight. All dyes will fade if exposed to bright light long enough, but sunlight may fade them a little more quickly after they have been treated with a commercial dye fixative.
hope this helps to clear up some issues