Am I supposed to use the white or green part of the scallion?

Both the green and white parts of the scallion are edible: the green has a milder flavor and makes an attractive garnish. Generally, the white part of the scallion is sharper tasting, or more onion-y, and is used when it will be cooked.

Recipe: Tuna Steaks with Grilled Scallions and Tomatoes