What is the primary difference between a red and green navigational light on a vessel?

The primary difference between a red and green navigational light on a vessel is that a red light indicates the port side while a green light indicates the starboard side. This color-coding system is crucial for ensuring safe passage and navigation during both day and night, allowing mariners to quickly identify the direction a vessel is facing and make informed decisions to avoid collisions.

In maritime navigation, the red light being on the port side means that when two vessels approach each other, the green light on the starboard side of a vessel gives them the right-of-way. This understanding is vital for maintaining safe distances and proper navigation in confined waters or during nighttime operations.

The other options do not accurately represent the purpose and significance of the navigational lights. Describing red lights as indicating stern and green as indicating bow, or labeling red as a caution light and green as a safe passage signal, obscures their specific roles in vessel identification and navigation. Likewise, mentioning that red denotes an emergency signal while green is for normal operations misrepresents their function, as these lights are not used for communication of emergencies but rather for indicating side orientation during navigation.