An avocado’s skin, with the perfect level of ripeness, generally appears deep, dark green, purply, or black, while bright green skin indicates the flesh inside is not yet ready.

Avocados, being climacteric fruits, continue to ripen even after they are harvested. As an avocado ripens, it undergoes a number of small, subtle changes in both texture and color.

These changes are prompted by an increase in respiration rate and the release of ethylene gas, which acts as a ripening hormone and triggers a cascade of biochemical reactions.

One of those reactions is the degradation of chlorophyll, which causes the fruit to turn black from green. Note that this applies to only some avocado kinds, namely Hass avocados.

Given that Hass avocados are by far the most commonly sold type in the U.S., selecting avocados at the supermarket based on their color is a safe option if you’re stateside.