What is required for the color of letters on registration numbers?

The requirement for the color of letters on registration numbers emphasizes that the letters must contrast with the background. This contrast is crucial for visibility and legibility, ensuring that registration numbers can be easily read from a distance or during various lighting conditions. Having a contrasting color helps distinguish the registration numbers from the surface they are applied to, thereby aiding law enforcement and other boaters in recognizing the vessel's identification.

The other choices suggest conditions that either do not ensure adequate visibility or impose unnecessary restrictions. For instance, blending colors with the background may hinder the visibility of the numbers, making them hard to see. Allowing any color without contrast could lead to similar visibility issues, and restricting the letters to only white would not accommodate a range of suitable color combinations that can provide the necessary contrast. Overall, ensuring that the letters stand out against their background is essential for safety and compliance with boating regulations.