How do I get rid of yellow stains caused by dark polishes?

Discolouration of the nail plate is generally caused by UV light reacting with the pigment in the polish so it is essential to wear a good base coat which acts as a barrier.  There are some things you can do to "cure" yellowing ..... after removing polish, you can lightly buff away the top discoloured surface of the nail with a multi way nail buffer. This will leave the nail plate fresher and cleaner underneath. 

There are also brightening treatments, such as Sally Hansen Insta-Brite Nail Whitener, which instantly lighten the nail plate removing the yellow appearance.