Why green color of chilly turns red when it ripen?

The previous answer is correct on the basis of physiology. The reason behind this is for good and efficient dispersal of the seeds inside the fruit. The maturity is attained and the seeds are ready to germinate and form new plants. For this a good dispersal mechanism is necessary. The red color of chilly makes it attractive and the biological agents like birds, animals etc are attracted and help for dispersal of seeds.