To colorfast noncotton fabrics, mix together 1 gallon of clean water, 1/4 cup of table salt and 1 cup of white vinegar in a large bowl or bucket. Gently stir the mixture to evenly distribute the salt and vinegar. Place the fabric or garment into the container, and press the fabric down to fully saturate it in the solution. Allow to soak for 24 hours. Note that some dye may bleed into the solution; this is acceptable.
After soaking, remove the garment, then empty and rinse out the container. Refill with fresh water, and add the garment back in. Gently agitate the garment to help rinse out any residual vinegar. If color continues to bleed out into the water, soak the fabric for another 24 hours in the vinegar and saltwater solution, then rinse in fresh water again. Allow the garment to fully dry before washing or wearing. In addition, to help prevent bright clothing from fading or transferring dye in the wash, use the washing machine's cold water setting and add 1/2 cup of white vinegar during the rinse cycle.
Vinegar may be less effective on cotton fabrics. To ensure colorfastness for cotton, use a cationic dye fixative such as Retayne. Before washing a garment for the first time, fully submerge the item in hot water and mix in 1 teaspoon of Retayne per yard of fabric. Allow to soak for 20 to 30 minutes, then wash the garment in hot water with regular detergent.