Final answer:
A green tube is used for an ammonia test due to the heparin additive that prevents blood clotting.
Step-by-step explanation:
A green tube is drawn when a ammonia test is ordered. The selection of the tube color is important because different colored tubes contain different additives which are chosen based on the test to be conducted. In this case, a green tube typically contains heparin which prevents clotting, making it suitable for ammonia and other chemistry tests where plasma is required.