How do you whiten coral?

I brought back some coral that had washed up on the beach from a vacation to Barbados a year ago. We bought a couple of glass rum bottles to put them in so we could display them in our home as a reminder of the trip. In one bottle, the coral has remained pristine and white. In the other bottle, the coral has turned a dingy gray color. Does anyone know how to whiten the coral up a bit?