The yellow discoloration that may develop on textile materials during processing, use, or storage.
Textile Resource (http://www.textile.org.uk)
A white fleece can yellow for a variety of reasons. Yellow stains can be caused by urine or feces, and by bacteria or fungus. Alkali and light can also cause yellowing in wool. Most stains cannot be removed by washing. Bleaching will probably damage the fleece
Textile Links (http://www.textilelinks.com)