from green peppers.. Purple peppers, too. that is why red (and other color) peppers are usually more expensive than green peppers-- they take longer to grow, and because they are fully ripe, they rot faster. ( Haven't you ever seen a green pepper with some red? )

peppers, unlike some fruit/vegetable will not ripen one picked. green tomato's can be gassed with ethanol?-- a naturally occuring gas that fruit gives off (apples give off a lot) to turn them red and "ripen" them-- of course they don't taste as good as vine ripened ones.. but a pepper picked at the green stage will not ripen into a red one... It has to do that before being picked.