Diesel exhaust fluid (DEF) is to be added to a new model diesel vehicle. The cap on the DEF reservoir/bottle is normally:

The cap on the diesel exhaust fluid (DEF) reservoir is typically blue to help identify it easily. This color coding is important as it prevents confusion with other fluids in the vehicle, such as engine oil or coolant, which may have different cap colors. This specific designation simplifies the process for vehicle owners and technicians when they need to add DEF, ensuring that the correct fluid is used without misinterpretation. Blue is the standard color that has been widely adopted in the automotive industry for DEF caps, aligning with regulatory guidelines and best practices for fluid identification in diesel engines.