If you have found that you are getting a yellow tinge on your nails it could be from your nail polish, especially if you are using cheap brands.
While there are medical conditions that make your nails discolor that you should have a doctors look at most of the time when you nails turn yellow it's from nail polish.
If the nails have been severely discolored then throw out the nail polish that you have been using.
Switch to a brand of nail polish that advertises a non yellowing formula and let your nails breath in between manicures with no polish on them. Then try this to whiten them again.