I'll try and explain as best as I can.
Keishi pearls are in fact a by product of cultured pearl production.
In the production of a cultured pearl, a small bead of pearl shell
and some grafting material is inserted into an incision made into the
flesh of the oyster. It's the rejection by the oyster of this
grafting material which inturn becomes a keishi pearl. At least
that's my understanding of how it is with the Pinctarda Maxima and
Margaritifera oysters here in Australia. The Keishi( sometimes spelt
kechis) can also be produced by the invasion of a parasite or worm.
The oyster senses that it's under attack and begins to cover the
invader with successive layers of nacre to reduce the irritation to
oyster. It's the basis of the above methodology employed to produce
round cultured pearls.
The colour of the pearl is determined by many factors, first being
the genis of the oyster and other factors such as nutrients and water
temp. Some pearls can be treated to change their colour. eg dyeing.
Trust that this is some help. Cheers