Leaves are normally green because of chlorophyll. Chlorophyll absorbs blue and red wavelengths of light, so it appears green. This molecule is crucial for photosynthesis but when the days begin to get shorter as winter approaches, plants begin to produce less chlorophyll. As the level of this green chemical in leaves falls, we can start to see other highly colored molecules. In particular, carotenoids, which appear yellow, orange, or brown, start to become visible. These molecules are always present in the leaves, it’s just that the green color of chlorophyll dominates most of the year.