Nail polish can discolor fingernails for several reasons, explains Self magazine. Color reactions from dark polishes are most common, with colorants reacting to the nail plate and causing yellowing. This reaction can be hard to predict, as not all dark polishes cause discoloration.
Additionally, formaldehyde, a common ingredient in nail polishes, can react with the keratin in fingernails and cause the nails to become brittle and yellow. An extreme yellow color may indicate an infection of the nail, notes Self magazine. To prevent nail polish from turning nails yellow, individuals can wear a base coat under polish and choose lighter polishes that are less likely to stain nails.