How do Tahitian pearls get their color?

To cultured a black Tahitian pearl usually takes up to 2 years. The overall development of your pearl is affected by even the most seemingly insignificant of elements.

The colors of the pearls
Tahitian pearls colors are not human made, they get their colors from the lip of the oyster they're generate, "Pinctada margaritifera". 

A pearl is formed from shell-forming material called nacre, which comes from the oyster's own shell. "Pinctada margaritifera" is black on the outer face, which is why the majority of Tahitian Pearls are darker than the other types of pearls when they are harvested. They take on the colors of the outer shell, which is why pearl growers examine the lip of the oyster to determine whether or not its pearls will also have a specific coloring if the oyster itself has that coloration. 

Luster of the pearl
The clarity of the water in which a pearl was grown can have an effect on its lusters. Because of this, even if the water in Tahiti is clean, they still have to go to neighboring islands in order to choose the most favorable location to make revenue.