The difference between green, red and yellow bell peppers
http://www.whfoods.com/genpage.php?tname=foodtip&amp;dbid=68

Same plant species, different cultivar, different maturity, different tastes, different nutritional value. To summarize from the link, green are harvested earliest and contain the least vitamins, yellows are next and contain more vitamin C and less vitamin A and beta carotene, reds are harvest last and contain the most each vitamin types. Yellow and red are both sweeter and more fruity than green.