Leaves are green because of a green part of the leaf called Chlorophyll. Chlorophyll is made up of even smaller parts called Chloroplasts. Chloroplasts have a green color to them. Leaves have Chloroplast because they help the plant or tree collect sunlight and turn it into sugar. That process is called photosynthesis. Chloroplasts are basically the tiny parts of cells that produce the plant’s food. Not all leaves are green. I am talking about when leaves die and fall off of trees or plants. that happens because there is less and less Chlorophyll as the tree or plant goes dormant. Then, they fall off because there is not a water supply to the leaf.