Don’t throw them in the tumbles, even soaking them in water too long
can turn them into mush. If the pearl is one color over the whole
pearl it’s dyed. If it’s under 8mm and black, it’s dyed, you can
look at the drill hole with a loupe, there may be a dye line, or
look at the surface with a loupe, if the color is darker around a
blemish, they dye may have pooled there.
I recently saw some Tahitians that someone got a great deal on. You
could see a layer of lacquer in the blemish. The string being
colored is fairly obvious. Rub them with a chamois, look at them in
sunlight on a white background. Pearls with a nice nacre should be
luminous with just the buff of a chamois.
The fact that some of your pearls are off color should be a sign
that they aren’t dyed. Buff them with a chamois and bee’s wax, make
sure you get all of the bee’s wax off. Maybe a little TLC will help
with the color.
As for changing the color, I’ve heard of people using everything
from Color Magic, a glass paint, to finger nail polish and being
happy with the results.
Wholesale/Retail Tahitian Black Pearls & Jewelry