One of these things is not like the other: red, orange, yellow, and green bell peppers . . . right? Not quite; while green bell peppers stand out for having a grassy, mildly bitter flavor, they're all the same species of plant — Capsicum annuum — and sometimes even the same cultivar. (Green bell peppers are immature versions of red, orange, or yellow peppers. One notable exception is the Permagreen cultivar, which is green when fully ripe.) This explains why green bell peppers are less sweet and fruity in flavor, and also why they're often less expensive, as they require less growing time.
What does this mean to you as a cook? While red, orange, and yellow bell peppers can be used more or less interchangeably, green bell peppers bring their own flavor to the table and shouldn't be used in recipes that call for other colors, as they are less sweet.