Bell peppers

As a bell pepper ripens, it turns from green to yellow, orange, or red, depending on the variety. These bright peppers are sweeter simply because they are ripe, whereas the bitter green pepper is unripe. Purple peppers, too, are harvested when immature and would turn an uncommonly dark green if allowed to ripen fully. So unless you're fond of the tannic bitterness of the common green and the purple varieties, we suggest sticking with yellow, orange, or red.

[Cook's Illustrated]