What engine fluid is indicated by a blue stain?

The blue stain indicates that the fluid in question is washer fluid. This type of fluid is typically brightly colored to distinguish it from other automotive fluids and is used in conjunction with windshield wipers to clean the windshield. The blue dye is added to make it easily recognizable, helping drivers to identify low fluid levels in the windshield washer reservoir or detecting any leaks.

In contrast, other fluids such as brake fluid, coolant, and transmission fluid usually have different colors; for instance, brake fluid is often clear or amber, coolant can be green, pink, or orange, and transmission fluid is generally red or brown. By knowing that washer fluid is specifically colored blue, drivers can promptly address any issues related to their windshield cleaning system.