You can harvest when the olive tree bears fruit. Contrary to popular belief, there are no black and green olive varieties. The color depends on the ripeness of the fruit.
Unripe olives are light green and firm. Their taste is very bitter and they have a high content of polyphenols, which make them very healthy.
When the olive begins to ripen, it turns yellow-green. At the end of the ripening process, it has the color dark violet/black.