When regular purple eggplant is fully ripe, it goes from having a deep purple color to having a light purple or brown color – unfortunately, it is also bitter when ripe. It is important to pick purple eggplant as big as they get but before it ripens and while they are still elastic and shiny and as soon as the deep purple starts to lighten or whiten and the top begins to turn yellow.
So if you see light purple or brown eggplants in the market, avoid them because they are overripe and are likely to be bitter in taste. A technically ripe eggplant is actually already passed its prime. It is a bit counter-intuitive compared to other fruits and veges, but eggplants have their own logic in this regard and tend to get lighter as they ripen.
I you want to know when is japaneese eggplant ripe, it’s the same deal as regular large purple eggplant. If your eggplants that have lots of white on them when immature, pick before the white turns yellow and then light brown.
If you want to err on the side of caution, follow the principle that it is better to pick eggplant slightly immature rather than too mature. Eggplants, like cucumbers, are best when their seeds are immature and therefore not bitter. So other than waiting for them to get as big as they can before you harvest, picking them a bit before they start to ripen is the way to go.