Why are leaves green?

The green color in leaves is caused by the presence of a compound called "chlorophyll," which the plant produces to do photosynthesis, which is how plants get energy from sunlight. The purpose of chlorophyll is to assist the plant in making food for itself by utilizing energy from sunlight and nutrients from the soil.

The chlorophyll is green because it absorbs red and blue wavelengths of light -- and so green is not absorbed and is reflected, making the plant look green to your eyes. Non-transparent things are the color they are because that is the color that is NOT absorbed -- the color you see is the color that is reflected... which is why you see it!