No, the darkness of a fruit or vegetable's color (called hue) is not necessarily related to its nutrient diversity or nutrient density. The reason is very simple.
A relatively small number of nutrients provide fruits and vegetables with their color. (The majority of these nutrients are pigments - flavonoids, carotenoids, melanins, porphyrins, and a few other categories of nutrients). But, more of a specific nutrient that provides color will make the food darker in that color.
However, most of the nutrients needed by the human body are not pigments and do not produce unique shades of color in fruits and vegetables. A food could be very light in color and contain large amounts of these non-pigment nutrients. A good example would be white onion - one of the lightest colored vegetables we can imagine! White onion is a significant source of vitamin C, vitamin B6, and folate, as well as chromium, manganese, molybdenum, phorphorus, and copper. It also contains a variety of highly unique and health-supportive sulfur molecules. Even with all of these important nutrients, however, it remains white.
Even when comparing different varieties of the same food - for example, green grapes versus red grapes, or a green Granny Smith apple compared to a Red Delicious apple - the differences in color do not usually represent "better" and "worse" in terms of nourishment. The skin of a green Granny Smith apple is going to have more chlorophyll than the skin of a Red Delicious apple, but the Red Delicious is going to have more anthocyanins. This difference is not a case of better versus worse - it's a case of being nutritionally unique, with each offering its own nutritional benefits. On our website we emphasize the principle of the unique nutritional value of each individual food. The diversities of color and intensities of color are important exactly because of this diversity.
We would like to mention one exception to the principles described above: processed food. Processed food typically loses its natural color because the natural food pigments that contain color are processed out. To compensate, the food manufacturers usually add artificial colors. When it comes to processed food, you might be in more trouble with a richly colored food than a pale, uncolored version due to this nutrient-depleting and artificial coloring process.
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