What condition must additional lights on a vessel meet?

Additional lights on a vessel must not interfere with keeping a proper lookout, which aligns with the principles of navigation safety and situational awareness at sea. The primary purpose of navigation lights is to communicate a vessel's position, status, and movement to other vessels, which is crucial for avoiding collisions.

If additional lights were to distract from or obscure the visibility of the required navigation lights, it could lead to misunderstandings among other navigators regarding the vessel's intentions and status. This is particularly important as vessels must maintain a proper lookout at all times to ensure the safety of both the vessel and others in the vicinity.

The other options suggest conditions that either do not adhere strictly to safety protocols or could create confusion. For example, prescribing additional lights to be brighter than the standard could mislead other vessels, while implying that they must blink or be any color could introduce further complications in communication at sea.