A lot of pearl farmers wish they had the full answer to that question. Then they could control the color of the pearls they cultivate. As it is, they have only part answers or clues, which include the following:

The type of host oyster
Oysters vary in their potential to produce certain coloured pearls. For example, Tahiti Cultured Pearls are cultivated in the black-lipped oyster because other oysters do not produce pearls of the same type. Even though pearl farmers know the black-lipped oyster is essential to the cultivation of Tahiti Cultured Pearls, they don't know yet how to consistently make it produce a specific colour. The pearl may end up being white or a variety of shades of grey as well as black, bronze, greenish or purplish.

The quality of the nacre
If the nacre is very thin, the color will look milky and lack overtone tints. Besides being affected by the number of layers of nacre, pearl color is affected by the thickness of each layer.

The environment they are grown in
It's theorized that there may be trace elements in the water that affect the color.

The color of the tissue that is inserted with the bead nucleus
This involves the color of tissue from another oyster's mantle (the part of the oyster that secretes pearl nacre) that has to be implanted with the shell bead for a cultured pearl to grow.

What color is Best for You?
Most pearl experts agree that a buyer's color choice should be primarily based on what will look good on the person who will wear the pearls. Some sales persons, however, give color advice by suggesting what's popular in specific geographical areas.
But you will also want to know how the color affects the price of pearls.