A variety of conditions can turn fingernails yellow.
Onycholisis occurs when the nail has been separated from the bed and can cause fingernails to appear whitish yellow. Fingernails can appear to turn yellow from skin and fungal infections or from underlying medical conditions. Rarely, yellow fingernails can be caused by an inherited condition called ‘yellow-nail syndrome', in which the nails don't grow at a normal rate and are discolored.
"Nail Discoloration." MedicineNet.com. N.p., n.d. Web. 11 Nov. 2001. www.medicinenet.com/nail_discoloration/symptoms.htm
"Nail abnormalities: MedlinePlus Medical Encyclopedia." National Library of Medicine - National Institutes of Health. N.p., n.d. Web. 14 Nov. 2011.