The green color of leaves is due to a pigment called chlorophyll. Most leaves have other pigments, too, but they're masked by the presence of chlorophyll. In late summer and fall, with decreasing day length, leaves produce less chlorophyll, and the other pigments become visible. The red and yellow underlying pigments are produced most abundantly during warm, sunny days followed by cool nights. Red pigments are produced as a result of sugar accumulation as chlorophyll breaks down. Low sugar production under poor conditions reduces pigment production. If cloudy, cool weather hits during early fall, colors are not as spectacular.