Prevention notes several reasons you could have yellow fingernails. First and foremost, it might just be simply because you wear a lot of dark nail polish regularly and it's staining the nail plate underneath. As dermatologist Rina Allawh, M.D., explains, "The dye in the nail lacquer interacts with the keratin of the nail, causing a yellow discoloration and brittleness." 

Acetone nail polish remover can actually worsen the situation, giving the dissolved polish more opportunity to bond to your nails. If you're concerned, lay off the polish for a while until the color returns to normal and always apply a clear base coat when painting your nails in the future. Likewise, if you work with your hands a lot, use a lot of fake tanning products, or smoke, then yellow discoloration may occur as a result of your lifestyle.