How should the letters on the "School Bus" signs be colored?

The correct coloring of the letters on "School Bus" signs is black letters on a yellow background. This color scheme is used to ensure high visibility and recognition, especially in various lighting conditions and distances. The yellow background provides a bright, attention-grabbing color that stands out, while the black letters offer a strong contrast that makes them easy to read. This combination is widely recognized and understood, helping to promote the safety of children as school buses are often present in low-visibility situations such as early morning or late afternoon.

This specific design standard helps ensure consistency and recognition across all school buses, aiding drivers in understanding the presence of a school bus and the need to exercise caution when approaching. The other options do not conform to the recognized standards for school bus signage, which is why they are not correct.