What factor does NOT determine the safe speed when going down a long steep downgrade?

When considering the safe speed for navigating a long steep downgrade, factors like the length of the grade, the weight of the vehicle, and the weather conditions all play crucial roles in ensuring safe operation. The length of the grade influences how much momentum the vehicle can build up, which can significantly affect control. Heavier vehicles require more distance to slow down, making their weight a critical factor in speed regulation. Weather conditions, such as rain or snow, also impact traction and stopping distances, which are important for determining a safe speed.

In contrast, the color of the vehicle does not have any impact on its speed, handling, or braking capability while descending a steep grade. Therefore, it is not a relevant consideration when assessing safe driving practices in such scenarios.