Final answer:
The color of the tube stopper that indicates a tube containing sodium citrate is blue. Sodium citrate tubes are used in coagulation studies.
Step-by-step explanation:
The color of the tube stopper indicating a tube containing sodium citrate is blue. This is a standard in phlebotomy, where color-coded tube stoppers indicate the type of additive contained within the tube. Tubes with a blue stopper typically have sodium citrate as an additive, which acts as an anticoagulant; they are commonly used for coagulation studies such as PT/INR and aPTT tests.