The chloroplast within eukaryotic cells (cells that contain a nucleus and organelles) contains a green pigment called chlorophyll. This chlorophyll is housed within the disc-like thylakoids of the chloroplasts. In green plant cells, chlorophyll is the most abundant plant pigment. However, there are other pigments that are present in lower ratios. For example, carotenoids are orange plant pigments. Xanthophylls are yellow plant pigments. Anthocyanins are red plant pigments. These secondary pigments are masked by the abundant chlorophyll during the growing seasons of spring and summer. At that time, the chlorophyll is used to collect sunlight energy during photosynthesis in order to convert carbon dioxide and water into glucose (a sugar that the plant uses as an energy source) and oxygen gas.
In the fall, there is less sunlight. This implies that the process of photosynthesis slows down. This, along with the lower fall temperatures, results in the breakdown of the chlorophyll. As the chlorophyll breaks down, the other, less prevalent, pigments show their colors.