Set the colors in fabric by washing the material in very cold water with 1/2 cup of iodized salt. The salt prevents the colors from bleeding. Add 1 cup of vinegar to the rinse cycle in order to further set the colors and prevent the colors from fading.
The chloride in salt helps seal the color into the fibers and keep it looking fresher for longer. The vinegar not only softens the fabric, but it also prevents detergent build-up, which leeches color. This technique works on a wide variety of textiles.
If the material seems to be losing a lot of color or has the potential to lose a lot of color, fill the washing machine halfway with water, and add 1 cup of both salt and vinegar. Mix well, and add the fabric. Let the fabric soak for one hour. If there is dye in the water, repeat the process until the water is clean when the fabric is taken out.
Do not use hot water unless washing out any excess dye. In some cases, the clothing is dyed correctly but has excess dye in it. Many clothes are not adequately washed during manufacturing, so there may be lingering bits of dye in the fibers that did not adhere properly. One wash in cold water and two washes in hot water with detergent removes any loose dye.