Plants are green in color due to the presence of chloroplasts inside plant cells. These chloroplasts contain chlorophyll, the molecule responsible for trapping the sun's energy as a part of the plant's photosynthesis. Chlorophyll's natural green color creates the plant's green color.
Chlorophyll is only one of several pigments used by plants to absorb light. Each plant shows the color of the pigmentation that best allows them to absorb sunlight based on their surroundings. During autumn, plants and leaves often turn orange or yellow in color. Due to the reduced amount of sunlight during the days, the amount of chlorophyll is reduced in the plant cells due to the lack of energy resulting in malnutrition. The lack of energy triggers the process of breaking down and re-absorbing chlorophyll, allowing other forms of pigmentation to show.