"The colors of vegetables are due to various families of pigments that can exist simultaneously in the same plant. The most important three families are chlorophylls,Carotenoids , and Flavonoids.
Chlorophylls exist in diverse forms. The most widespread are chlorophylls a and b. *They possess a porphyrin ring that contains a magnesium atom in the center,
*and a long chain with 20 carbon atoms is grafted onto this ring.
*They absorb sunlight at wavelengths shorter than 700 nm.
*However, the efficiency of absorption is weak around 530 nm, and in this wavelength region, light is sent back by diffuse reflection (that is, in all the directions), which provides to our eyes a general sensation of green.
* The various nuances of green result from the presence of several types of chlorophylls and other pigments, such as carotenoids and flavonoids, which contribute a little to the color.
Carotenoids, which are very widespread in the world of plants, provide yellow, orange, and red colors. In this family, we can distinguish between carotenes and xanthophylls. *The first consist only of carbon and hydrogen atoms,
*whereas the second contain oxygen atoms in addition.
*The leader of carotenes is β-carotene, which is responsible for the orange color of carrots. As for tomatoes, they owe their red color to lycopene. The most important of the xanthophylls is lutein, which is yellow. It can be found in corn and especially in the green leaves of vegetables (e.g., spinach, sorrel, broccoli, cabbage), even if its color is masked by chlorophylls.
Flavonoids are especially present in flowers. They are flavonic compounds that belong to the polyphenol class.
*The skeleton consists of three cycles (two benzene rings and an oxygenated heterocycle) that bear hydroxyl groups (OH) in various positions.
*They can be divided into several groups of compounds, in particular, flavanones, flavonols, and anthocyanidins.
Flavanones and flavonols give various shades of yellow.
Anthocyanidins undergo a glycosylation by an enzyme within plants, which leads to anthocyanins. Their colors strongly depend on the number and on the position of the hydroxyl groups: red, pink, blue, violet, and purple.
These colors can be affected by complexation of metals or by changes in acidity. For example,
*the color of cyanidin is blue in basic medium (e.g., cornflower),
*violet in neutral medium,
*and red in acidic medium (e.g., poppy).
--Red cabbage also owes its color to cyanidin. Its juice is blue in basic medium and red in acidic medium. Anthocyanins are also responsible for the color of dark red fruits (e.g., blueberries, blackcurrant, black grapes)." ChemistryViews