Final answer:
The color stopper tube used to collect a blood specimen for a prothrombin time test is light blue. It contains sodium citrate to prevent blood clotting.
Step-by-step explanation:
The color stopper tube used to collect a blood specimen for a prothrombin time test is light blue. The light blue stopper indicates that the tube contains sodium citrate, which is an anticoagulant that prevents the blood from clotting.