By "the white part", I am assuming this to be the free edge of the nail plate as it grows past the end of the fingertip. If this is the case, it is not the nails that have necessarily yellowed, but the solehorn cuticle that grows out with and underneath the nail plate at the free edge. Try soaking your nails in 5 minute Polident Denture Cleaner. When the nails and skin are significantly softened, use a cuticle pusher with a rounded head to scrape away all the 'dead' tissue that is attached to the underside of the nail. I think you will be VERY suprised at the amount of dead skin under there, as well as how bright the nails are after it is removed. It is the dead skin that gets discolored and looks 'yellow'. The nail plate can also take on a yellowish appearance if Base Coat is not used prior to the application of nail color.