Yellow Fingernails

Try soaking your nails in lemon juice for about five minutes a day. If it doesn't clear up in a few days, it may be a fungal infection. You can find anti-fungal treatments in the nail care section of most drugstores. It's just a liquid that you brush on like nail polish. However, fungal infections can be quite stubborn and if it doesn't clear up I would see a doctor as you may need prescription medication to treat it.

Either it's an infection or a nailpolish you used.

If you wear nail polish a lot, put on a base coat first before you apply polish. Sometimes yellowing can be contributed to aging, you can give your nails a light buff with a nail buffer this may remove some of the yellow color.
Soak your fingernails in lemon juice daily or soak in lemon essential oil, or you can dissolve a denture tablet and soak nails.

presoaking you nails in white vinegar help strengthen and lighten nails