Chloroplasts contain chlorophyll which is what makes plants green. Onions are not green because they do not contain chloroplasts. The reason they do not have chloroplasts is because in the chloroplast, photosynthesis takes place. A key player in the process of photosynthesis is the sun. The onion itself grows underground which means while they grow they do not come into contact with the sun so chloroplasts in an onion is pointless since they can’t undergo photosynthesis. Hope I helped!
