The stem and leaves are green because of the presence of green pigment chlorophyll which absorbs the sunlight and performs photosynthesis.
Explanation:
The green colour of the parts pf the plant above the soil is due to the presence of chlorophyll pigment which lives in chloroplasts of the leave cell. This is characteristics of the plant.
When chloroplasts come in contact with sunlight it absorbs the light and photosynthesis takes place in which carbon dioxide and water reacts to form glucose. This is the reason plants are called primary producers.
The roots are responsible for absorption of water and inorganic minerals, adhering plant to the soil, stores food and transporting the nutrient and water by specialized water conducting cells as tracheal and vascular tissues, xylem.
The roots lack the chloroplast and the green pigment hence have no green colour.