The color of the olive corresponds to how ripe they are when picked, in addition to the curing process they undergo.
Green olives are picked before ripening, and black olives are picked while ripe, which is when the color has turned from green to black.

Raw and freshly picked olives are inedible due to their very strong bitter flavor, so both green and ripe varieties are cured, either by being packed in salt, brine, or water, before being eaten.

Generally, green olives are denser, firmer and more bitter than black olives.
But the taste and texture of any olive depends on the method and duration of the curing process.