Biologists tell us that color is the visual effect on light through eyes, brain and our life experience. The light we see with the naked eye is produced by electromagnetic waves with a narrow wavelength range. Different wavelengths of electromagnetic waves stimulate different cones to different degrees, and different signals show different colors under the processing of the brain. In other words, color is a kind of visual nerve feeling produced by the naked eye under the stimulation of electromagnetic radiation energy. Leaves turn yellow because chlorophyll breaks down in winter, leaving yellow carotenoids as hydrocarbons. Carotenoids are photosynthetic pigments in chloroplasts, including carotene and lutein. I believe everyone is familiar with it. Red leaves are caused by red anthocyanins in vacuoles. Anthocyanin is a water-soluble pigment, which can change color with the pH of cell fluid. Cell sap is red when it is acidic and blue when it is alkaline. Anthocyanin is one of the main pigments that constitute the color of petals and fruits. Anthocyanin is a secondary metabolite of plants and plays an important role in physiology. It is commonly found in the tissues of flowers and fruits, as well as in epidermal cells and lower epidermis of stems and leaves. The acid-base indicator purple litmus test solution uses anthocyanins. How about a yellow apple? This is because ferrous iron is oxidized into trivalent iron, which reflects the color. Remember when the middle school teacher emphasized that ferrous ion is green, ferric ion is yellow and copper ion is blue? Actually, that's not true. Most of the time, the simple ions of metal elements are colorless, and their complexes can reflect many colors. This is because coordination changes the energy level of metal ions, making them in the visible range, which is the color of ions we see.