Freshwater pearls don't come naturally in black, gray or gold colors. Those pearls have been dyed. If you ever see freshwater pearls in green, blue, red, garish purple-- they have also been dyed.
Natural colors are variations of white/ off white, peach, pink, lavender. Sometimes there are fancy overtone colors as well.
Freshwater pearls are grown in very large quantities and do not have much value if you are thinking of reselling. But it doesn't make them any less beautiful to wear