What is the appropriate song to be played on the bugle when music is not available for evening colors?

The appropriate song to be played on the bugle when music is not available for evening colors is Retreat. This call serves to signal the end of the duty day and is typically sounded at sunset, coinciding with the proper protocol for the lowering of the national flag. Retreat helps in creating a solemn atmosphere and marks the transition from day to night, similar to how formal ceremonies are observed.

In contrast, Taps is traditionally played at military funerals and memorial services, not specifically for evening colors. Reveille is associated with the morning hours, signaling the start of the day and the raising of the flag, while the Star-Spangled Banner is the national anthem and is played during flag ceremonies and specific events, but not specifically for the lowering of the flag in the evening colors context. Thus, Retreat is the correct choice as it aligns with the established customs of military ceremonies.