When regular old purple eggplant is completely ripe, it goes from having a deep purple shade to having a light purple or brownish shade. Sadly, it is also bitter when even a little over-ripe. It is very important to pick deep purple eggplant as large as they grow but before it ripens too much. Another sign that they haven’t become too over-ripe is they still have a shiny and elastic surface to them and the deep purple is only beginning to lighten slightly and top is only beginning to turn yellow.
So if you see light purple or brown eggplants in the supermarket, reject them due to the fact that they are overripe and also are most likely to be bitter in taste. An actually fully ripe eggplant is already passed its prime. It is a bit counter-intuitive compared to other fruits and vegetables, but eggplants have their very own logic so tend to become lighter as they ripen..
If you want to know when a Japanese eggplant is the same principles that apply. If your eggplants are fully white or have white stripes on them when immature, pick them prior to the white turning yellow and light brown.
If you want to be careful about it, follow the concept that it is much better to pick eggplant a little immature as opposed to fully mature. Eggplants, like cucumbers, are best when their seeds are immature and for that reason not bitter. So besides waiting for them to get as big as they can prior to you picking them, pick them a bit early rather than too late.