Work on the discoloration by washing the fabric in the hottest water possible for the fabric that you are working on. Add a cup of water softener or two cups of vinegar to the water when rinsing the fabric.
Do not dry the fabric again until you have all of the discoloration out of the fabric.
Wash the fabric again, using detergent and either water softener or the vinegar again. Let air dry to check the real coloring of the fabric.
If the discoloration remains, check the tag to see if you can use bleach on this fabric.
Soak the fabric in a gallon of water with a cup of bleach for about ten minutes. Rinse well and let air dry to check real coloring of the fabric.
Repeat the above steps until the fabric is lighter, brighter, and more to the original coloring.
If the discoloration still remains, try using a color stripper to take all color from the fabric so you are left with a white fabric and then you can re dye the fabric to the coloring of your choice.