The colour of an olive indicates its ripeness. The darker the olive, the riper it was when it was plucked from the tree. Olives transform from green to light pink to red and finally black.
Green olives are usually picked at the start of the harvest season. They have a firm texture and lovely, nutty flavours. Darker olives are picked towards the end of the season and are softer, richer, and meatier.
Green olives yield high quality oil but a low quantity. Matured black fruit yield a high quantity but the oil is of a substandard quality. The best quality oil comes from fruit that is half-ripe, they produce a slightly lower oil content but of a superior quality.
When you want to process olives for black table olives you would harvest the fruit when the skin is completely black.