2 Answers

If you see: Yellow nails 
It could mean: Diabetes 
Discolored nails may simply indicate an infection, but if you're also making extra trips to the bathroom, constantly reaching for your water bottle, and feeling fatigued, a likely culprit is diabetes. The condition can cause glucose to attach to collagen proteins in the nail, turning them from pink to yellow. 
What to do: See your doctor for a diabetes test. If it's positive, he can recommend a combination of lifestyle changes and medication to keep the disease under control. 

Read more: http://www.oprah.com/health/Sign-of-Diabetes-Thyroid-Symptoms-Yellow-Nails#ixzz2OI3yRcBy

may be some liver involvement too. ......or a fungus.   Some people who have always smoked heavily have yellowish fingernails and fingers.