Final answer:
The color that signifies a health hazard in the NFPA 704 Marking System is blue.
Step-by-step explanation:
The color that signifies a health hazard in the National Fire Protection Association 704 Marking System is blue. Within the NFPA 704 diamond symbol, the blue diamond indicates the level of health hazard associated with a substance. This color is used to provide safety information about certain substances, indicating the potential health risks associated with their use or exposure.