My point in posting it was to show the blue-yellow markings below the course deviation indicator.
Thread hijack alert.
Blue/Yellow LOC references were included in early jet transport avionics systems.
The FD mode selection and feedback loop were central to the investigation of a
DC-9 accident at Boston July 1973.
In the report, one can find references to a "Blue Left" Sperry Flight Director mode:
As aterpster has noted, the CDI is turned upside-down by an HSI to provide proper orientation for the crew.
The BC (nee "Blue Left") FD mode is necessary to obtain correct steering from the FD.