What marking color is used to indicate if a cover glass has slipped?

The correct choice for indicating if a cover glass has slipped is blue. Instruments in aircraft are designed with various color-coded markings to provide quick visual cues to pilots and maintenance personnel regarding the status and functionality of the instruments.

In the context of aviation instrumentation, blue markings are specifically used to signal that a cover glass has moved from its proper position. This helps in quickly identifying that there could be an issue with the instrument's accuracy or readability, facilitating prompt attention to maintenance or recalibration to ensure safety and reliability during flight.

Understanding the color coding used in aircraft instruments is crucial for effective inventory management and operational safety, as it allows for immediate recognition of potential problems without the need for in-depth inspection.