According to About.com, the best way to set the color in jeans is by washing them inside-out in cold water and adding distilled white vinegar to the rinse cycle. If the jeans bleed, wash them alone and never with light-colored clothing.
Generally, colored jeans need to be washed before wearing. Turn the denim inside out and wash them in cold water. Add a cup of white vinegar to the rinse cycle to help seal the color in the denim. Certain jeans may continue bleeding color after washing. Typically, it is best to wash jeans several times to remove excess dye. Another option is soaking the jeans in vinegar for several hours and then washing them in cold water.