Yes. Red bell peppers are the sweetest because they are the ripest; green, yellow, orange and red peppers are the same fruit picked at different stages of ripeness.
Every bell pepper starts out green because it is full of chlorophyll. As it ripens, the chlorophyll breaks down and is replaced by yellow, orange and red pigments called carotenoids. Green peppers are ready about eight weeks after planting and taste slightly bitter and crunchy. Yellow and orange peppers are picked roughly two and three weeks later and have a mild, sweet flavor. Red peppers stay on the plant about four weeks longer than green ones, so they are the sweetest, richest in flavor and softest when cooked. Color also changes nutrition: orange and red peppers have the most vitamin C and beta-carotene, and red peppers have the most capsanthin, a strong antioxidant. The popular idea that a pepper with four bumps on the bottom is a sweeter “female” is a myth, because the number of lobes depends on the plant’s genetics and does not affect flavor.
Image: Red, yellow and green bell peppers, Nikodem Nijaki, CC BY-SA 3.0, via Wikimedia Commons.