If you have a piece of fabric or clothing that is light colored, pastel or white, sometimes it can look a little yellow. Light colored fabrics are sometimes manufactured with a special whitening agent that can break down after continued wear and wash.
Yellowing is one problem you are not going to be able to correct. You can try re-dying the fabric with a white wash or a white dye. Name brands vary from area to area but you can find this in any home center. Be sure to read the instructions on the packages.
