Life of an Olive: Almost Ready to Pick
The olives gradually turn black on the outside, then on the inside, and some time in the Fall, are ready to pick. The more black they are, the higher the oil content is (but the oil is not quite as healthy, and has a shorter shelf life due to a smaller amount of antioxidants). The oil from greener olives is has a more bitter taste, due to the antioxidants, yet can be quite delicious.
Some olives mature faster than others, as you can see in the two photos below.