If you suffer from yellowing nails, be sure of the following.  1)  Base coat is key (the pigments in nail polish can penetrate and discolour nail layers.)  2) Be sure to remove every trace of old manicure before applying new polish.  3) Avoid products containing formaldehyde, which can turn nails yellow.  (There are lots of brilliant '3-free' and even '5-free' polishes out there.)