There is a connection between cool temperatures and fall color, but frost isn't necessary. Most plants produce a variety of pigments all summer long, but for most of the year green chlorophyll is produced in such abundance that it masks any of the other colors. As the days grow shorter and nights get cooler, chlorophyll breaks down faster than it is produced by the plant. That allows other pigments-yellow xanthophylls or carotenoids and reddish-purple anthocyanins-to take over. Brightest colors develop when sunny days are combined with cool nights. In fact, a hard frost dulls fall color by turning the leaves brown more rapidly.