Similar to paper, old fabric can turn yellow with time. Unfortunately it isn't a pretty yellow that one would want on their fabric, but rather a stale appearing uneven aged yellow that makes the fabric appear unclean. Yellowed fabric doesn't mean that the fabric is ruined as it is easily reversed. It's a simple process that takes hardly any time at all to restore fabrics back to the colour they were meant to be.
Set the washing machine to hot or to the hottest temperature of water that the fabric can handle.
Place the fabric in the washing machine with a cup of vinegar. Do not use detergent.
Add a second cup of vinegar to the rinse cycle.
Check the fabric when the washing machine is finished with the cycle. If the yellow is still there, rewash the fabric the same way a second time. Continue to wash until all of the yellow is gone.
Hang the fabric outside to dry in the sunshine. This will allow you to see ensure that the yellow is still gone even as it is drying.
To keep from damaging the fabric by repeated washing: If the yellow doesn't come out by the second wash, soak it in a weak solution of one cup bleach to one gallon of warm water for at least 10 minutes before hanging it out to air dry.