The color purple in plants is mainly caused by the presence of pigments called anthocyanins. Anthocyanins are water-soluble pigments that belong to a class of compounds called flavonoids. They are produced in response to environmental stressors such as high light intensity, cold temperatures, and nutrient deficiencies. Anthocyanins are responsible for a range of colors, from red to blue, purple, and black. The exact shade of the color produced depends on the pH of the cell sap, the presence of metal ions, and other factors. The color purple is rare in nature because the molecular structure of anthocyanins requires specific conditions for its formation, and these conditions are not always present in plants.
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