Which pepper is most nutritious? Often times you hear people say it's yellow, orange, or red, which inconveniently, are also much more expensive than green bell peppers. Technically speaking, that answer is only half correct.
It is true that yellow bell peppers are the most healthy when you rank them based on antioxidants. However the amount by which they beat the other two colors is not significant. Furthermore, the rest of the advice everyone gives you about red vs. green bell peppers is not entirely accurate.
If you compare the nutrition of red vs. green, you will see their ORAC values are 821 and 935, respectively. That means it's actually green which is 14% healthier, at least by the measure of total antioxidant content.
Next, put head to head green versus orange bell peppers. In this match, green does lose but only by a hair; it is 935 versus 984 for the orange. Being that the latter usually cost twice as much, you would hope for that number to be twice as high, right?!
Now when you match up yellow bell peppers vs. green, the former is only about 12% higher than the latter.
The lesson? Supermarkets know that the fiery-colored varieties are perceived as being significantly healthier and they charge to reflect that. Don't feel like you need to pay up just to get decent antioxidant value from your peppers. That being said, taste aside, there is a situation where you gain more health benefits from the yellow, orange, and red. When? Even though all are pretty close for ORAC, if you evaluate them for specific nutrients like vitamin C and A, the green comes in lower. This is because the green are picked from the vine earlier, before they have fully ripened and developed their nutritional content. Also, the specific types of antioxidants in the fiery-colored versions include anthocyanins, lutein, zeaxanthin, and others which are found in much lower quantities of any green colored vegetable. For some health conditions, especially those involving the eye like macular degeneration, it's often advised to consume higher amounts of those specific types (ask your doctor).
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010