The colour of an olive that you get from the shop actually has nothing to do with what type it is: Olives are green when they’re plucked unripe from the tree and will change colour as they become increasingly ripe, from reddish brown to dark violet or jet black.
Unripe green olives are firm and taste more mild, while black olives have a more herbaceous note. But whether ripe or unripe, olives are extremely bitter when taken straight from the tree.
Most have to first spend months in a salt brine in order to be relatively enjoyable, with some further doused in oils, vinegars or herbal stocks, and others filled with almonds or refined with ingredients like garlic or chilli peppers.
Some olive producers bypass the long ripening process and dye unripe green olives with approved colour stabilisers such as iron gluconate or ferrous lactate. Dyeing them black is legally allowed but must be obviously noted on the ingredients list. Consumers can recognise a true black olive by the pit, which is also dark in colour. – dpa