Final answer:
The diesel exhaust fluid cap is typically bright blue to easily distinguish it from other vehicle fluids and prevent the addition of incorrect substances to the system.
Step-by-step explanation:
The color of a diesel exhaust fluid cap is typically designed to be easily distinguishable to prevent confusion with other fluids in a vehicle. For most vehicles, the cap is a bright blue color, which stands out against the engine bay and other component colors. This is to ensure that only diesel exhaust fluid (DEF), which is a urea-based solution, is added to the DEF tank, avoiding potential damage to the vehicle's system from incorrect fluid being added. While the fluorescent green-dyed antifreeze is visible in the radiator header tank, the DEF cap will generally be blue to make sure it is not confused with the antifreeze or any other vehicle fluids.