Fruits and vegetables are thought to provide health benefits because of the plant chemicals they contain. Some plant chemicals are bioactive in humans and are of interest because they have been shown to reduce the risk of a number of major disease, including cardiovascular disease and cancer. However, the physiological effects of these compounds is diverse, reflecting their structural differences, and so a wide variety of compounds would be consumed for optimal protection from disease. Generally plant material is green because it contain chlorophyll, the plant pigment that allows the photosynthetic process to occur. However, accumulation of plant compounds such as flavonoids, carotenoids, terpenes and stilbenes in fruits and vegetables can provide them with characteristic colours. This is because different groups and classes of phytochemicals absorb and reflect light in differing wavelengths, and thus specific compounds reflect characteristic colours.
Fruit and vegetable colour can be used as a general indicator of the type and amount of plant chemicals present without the need for complex chemical analysis. For example, anthocyanins, a class of flavonoids, are common in berries and give them their purple, red and pink colours. Carotenoids give peppers their yellow, orange and red colours and also produce the orange colour in carrots and red colour in water melons. Consuming a limited variety of fruits or vegetables only provides a limited number of phytochemicals. However, without an in depth knowledge of the chemical composition of various plant foods it can be difficult to ensure that a variety of phytochemicals are being consumed. Including a variety of coloured fruits and vegetables is therefore good advice in order to increase intake of a variety of possibly beneficial phytochemicals with differing physiological effects.