What plant part is not green?
The roots of a plant are not green.
A part of a plant is green because it contains chlorophyll, which is used for photosynthesis. Since no light penetrates the ground to reach the roots, the roots need no chlorophyll, and so are brown instead of green.
It is the green parts of a plant that trap the Sunlight and make the food (sugars) for the plant (in structures called "chloroplasts" which contain the green "chlorophyll"). The green parts of a plant are normally the leaves and the plant arranges these so that each leaf is exposed to the most sunlight. The roots transport water and nutrients to all parts of the plant, they do not produce food.