Red bell peppers are sweeter and fruitier, while green bell peppers have a slightly bitter and grassy flavor.
The difference in flavor between red and green bell peppers comes down to their stage of ripeness. Green bell peppers are harvested before they are fully ripe, which is why they have a less sweet and slightly bitter taste. As the pepper matures, it turns yellow, then orange, and finally red. During this ripening process, the pepper develops more sugars, resulting in the sweeter flavor characteristic of red bell peppers. The chemical compounds responsible for the pepper's flavor also change as it ripens, contributing to the distinct taste profiles. Red bell peppers have a higher concentration of capsaicinoids, though still very low compared to chili peppers, which contributes to their subtle sweetness.
When substituting bell peppers in a recipe, consider the flavor profile. If a recipe calls for green bell peppers and you only have red, use a smaller amount of red pepper to avoid overpowering the dish with sweetness. Conversely, if a recipe calls for red and you only have green, consider adding a pinch of sugar to compensate for the lack of sweetness.