yellow pepper, a study

Green, sweet bell pepper have 2 times as much vitamin C as oranges; red and yellow bell peppers have 4 times as much.

Green bell peppers are fully developed, but not ripe. All sweet bell peppers start out green, and change color as they ripen. (They also get sweeter). What can be confusing, is that there is also a variety that is green when ripe. The color depends on the specific variety, the most common variety is red when ripe. So...green sweet bell peppers start out green and are green when ripe. Red, yellow, etc. sweet bell peppers start out green and turn red when ripe. What you find in the store are usually either unripe green sweet bell peppers or unripe red ones. ~FoodReference.com