Living things need energy to stay alive. Plants get energy from the sun, and animals get energy from eating other plants or animals.
Plants contain chlorophyll, which absorbs sunlight to keep them alive. They appear green because chlorophyll absorbs only red and blue light from the sun. Because green is not absorbed, it passes through the leaf o is reflected back.
Besides being valuable for human nutrition, and plant chemical processes, carotenes provide bright colors in vegetables such as carrots, tomatoes and peppers.
Usually, there is more chlorophyll than carotene so the green covers up bright carotene colors.
When winter nears, plants begin preparing for dormancy, similar to hibernation. Before losing their leaves, plants stop producing chlorophyll.
When the leaves lose all their chlorophyll, the carotenes remain in the leaf and cause the bright colors that we see in the fall.