© Saidin Jusoh/Fotolia
Most tomatoes are red when they are ripe, because of the presence of lycopene, which is a phytochemical (a biologically active compound produced by plants). Immature tomatoes are rich in chlorophyll, another phytochemical, which gives them their green colour. As the seeds inside mature, the fruits ripen, undergoing a number of chemical and physical changes that render them soft, flavourful, and (usually) red. The colour change occurs as the chlorophyll breaks down and lycopene is produced, which signals to animals, including humans, that the fruit is ready to eat. Fascinatingly, tomato seeds can survive passing through an animal's digestive tract, so the animal-attracting red colour is part of the plant’s seed-dispersal strategy.